I want to preface this by saying that I'm not trying to argue with you. Characters can be interpreted in multiple different ways and that's cool! These are just silly little guys that we like to play with! No ones in the wrong!
But as an autistic myself, I want to offer a slightly different take.
First off, I think you are completely right in saying that she wasn't written to be explicitly autistic! Or at least I've never seen any sort of confirmation from the original author saying so. And I agree with that her childhood trauma contributes heavily to her social issues! Maomao says so herself!
But! Its a couple extra things that makes her come across as autistic (rather than solely emotionally repressed) to me personally. Or at the very least why she resonates with my experiences as an autistic.
First off, there's her love of medicine. It goes beyond what one might call a normal interest. To me it feels like it's part of her core in a way that's hard to describe. It's not just a job or a hobby or an interest in the way other people might have them. Its a part of who she is. Which is how special interests feel and it's very hard to describe that experience to someone who doesn't experience it.
There's also her family. Her biological family is made up of "weirdos". And correct me if I'm wrong but I don't think there's any special traumatic reason that Lakan can't see faces. Or Lahan's so obsessed with numbers. Autism runs in families. And stuff like face blindness is more common in autistic folks. It isn't a stretch to say that Maomao get some of her "weirdness" from Lakan.
There's also things like how she struggles with interoception (her pain senses being dulled) or how she can hyperfixate on problems or how she struggles to focus on things that don't interest her or her jumping up or down making strange noises when she's excited (stimming) or her sense of justice often overriding her respect for authority etc etc etc
And then there's her social issues. I'm not going to get too deep into them, because they're what most people will point to when talking about interpreting her as autistic. And of course you're right in saying that more than just autism can cause social issues and her less than ideal childhood probably greatly added to her struggles with that. But, personally, they really resonate with how I struggle socially.
I think it's a bunch of little things about Maomao that make her resonate with autistic folks. I don't necessarily think she's "100% canonically autistic" and I think it's a bit misleading to say that she was 100% written to be that way. But I can also see how its become a popular headcanon.
It's similar to how I think her struggles with romance and sexuality come across as very aroace-spectrum flavored to folks who are aroace. Do I think she was 100% written to be canonically aroace? No. I think if she was she would be a poor representation because she's pushed by the other characters away from that. But she resonates. Her complex relationship with those things feels similar to the struggles people who are on the aro and ace spectrums face. But its also misleading to say that she was written to be aroace because she wasn't.
(tldr: she probably wasn't written to be autistic, but more than just her social struggles resonate with autistic people, making it a popular headcanon. However, it may still be misleading to say "she's canonically autistic")