I think it's a little trickier than this when it comes to political argument. The questions (for me) are, "who is RFK speaking to when he says this?" and therefore "who do we have to speak to?"
Tl;dr: it's about the difference between emic (in-group) and etic (out-group) perspectives discussion of autism. If we want to persuade people who know very little about autism, we have to meet them where they are and argue ad hominem (in the proper philosophical sense of tailoring your arguments to your audience, not in the sense of insulting them). It's also not about arguing with fascists and eugenicists directly, but about trying to diminish their credibility in the eyes of the much larger group of people who are kinda ableist but not committed and could be swayed either way.
The reason RFK makes the claim that autistic people will never work, date, pay taxes, etc, is above all as a fear tactic. He is not making this point for the committed eugenicists and fascists out there. His audience for these statements is largely older people who are probably not engaged with or aware of the autism advocacy movement, and he wants them to be afraid of autism. When they hear "1 in 30" — there is a reason he's been harping on about this statistic — he wants them to think that one in thirty children have extremely high support needs and comorbid intellectual disability, and he wants them to be terrified for their children and grandchildren.
And they will be terrified, because honestly high support needs autism (the only kind they're generally aware of) can put a lot of pressure on a family, and I can't think of many countries where this pressure has been sufficiently alleviated by the state. I know some autistic people with incomparably higher support needs than me in my community, and it is really difficult when you get to a stage where their parents are too old to care for them. I know kids making decisions that cramp their future life choices, because it has been explained to them that they need to stay in town to look after their relatives because granny can't take care of them by herself these days. I do understand the fear that your child will not cope without your support, and that one day you will die and you don't know what will happen to them. Do I think the state should offer more support, so that noone should ever worry what their children will do without them, and so that people with high support needs can live with dignity as independently as works for them? Yes. But in the scare tactic works, because the world I would like has not materialised yet in most countries.
His audience are aware of high support needs autistic people as a concept, because for a long time they were what "autism" was used to refer to. Many of them grew up in a world where disabled family members were systematically hidden away and institutionalised (which does happen today, but much more rarely) and in which you never admitted to having a disability or mental health problem yourself (a norm which was understandable, since there were no laws against disability discrimination.) Others were raised by these people and are maybe simply not very curious or critical thinkers. They exist in a mental model of the world in which being disabled is a tragedy and at some level a shameful thing.
They are not, broadly, aware of low support needs autistic people. They are the people who say "oh, that person can't possibly be autistic, they're so lovely, they're just a little eccentric" and they say that because when they were learning social mores that was the kind thing to say. They often don't really believe that low support needs autistic people are actually autistic, because autism only means one simplistic thing to them, because that's what ignorance is.
These are the people RFK is speaking to. These are also the people we have to speak to, in my opinion. Because they are a larger group than autistic people and the people who are aware of the autism advocacy movement combined. Their views shape how they teach their children and grandchildren (it's a group with a broad age range) and it shapes how they behave towards the disabled people they know, and it shapes how they vote.
The views of these people matter, and they aren't — broadly speaking — awful people. They don't — usually — have any inherent hatred of disabled people, and they probably think of themselves as kind people who care about the less fortunate. But they fear statistics about rising rates of disability as an existential threat, and a large part of that is because they do not understand the vagaries of diagnostic criteria and how these have affected the statistics around autism.
And this ignorance matters enormously, because the fear it causes makes people who are not necessarily eugenicist in their current belief system (indeed many of whom are anti-abortion and believe more firmly in the "right to life" than I do) extremely vulnerable to eugenicist talking points. The ignorance has to be addressed.
It is extremely important that this large group of people learn to be less afraid. It would be very lovely if they learned to be less afraid because they understood the inherent value of disabled life, no matter how severely disabled. (Which actually some of them do, but that doesn't necessarily stop the anxiety of "who is going to look after these people whose lives have inherent value.") But it is more effective, in my experience, to reduce fear by exposure to the diversity of what autism means, showing where the propaganda oversimplifies things.
The internet skews this in the other direction: low support needs autistic people are loudest and most visible, which marginalises medium and high support needs people in their own community. That's awful. But that really isn't the case in the offline world, where autism is marginalised full stop and the only autistic person someone may be aware of is "Suzanne down the road's son who's forty and can't talk and I heard he has these violent fits and hits her and she had to call the police for help last week, everyone was talking about it". That person isn't aware of the concept of different levels of support needs. For them, autism means high support needs. Suzanne's son has a right to live a good life (and he and Suzanne might need better support than he's getting if he's having regular violent meltdowns) but that person's fear of 1 in 30 kids growing up to be like him is understandable, because that would be a huge amount of pressure on families and state support services. It's important for them to know that that is not the only thing autism means.
For many many people I have spoken to about autism, the fact that I have an official diagnosis and also have several degrees and a full time job and a home I pay the rent for and take care of by myself* is fucking groundbreaking new information. It opens up their worldview. It makes them less afraid, and more able to question the fear tactics and dehumanisation. That matters.
*Which, don't get me wrong, is a struggle. I am disabled. But I can more or less manage it, and for many many people the idea that I can do that and also be disabled is genuinely news.