"but isn't it kinda hypocritical to say that about someone while literally assuming their own emotions, despite the other person also being a stranger to you?"
you could see it that way if you took an absolutist stance on it, but I hate absolutism (for the most part). for me, I'm deducing this based on multiple other assumptions I've guessed, that I have then seen direct evidence of being correct. I can extremely easily predict the other person's behavior (down to nearly word-for-word what I think the next response will be), yet the other person fails to predict mine the vast majority of the time. every time they have tried to guess my emotional state and my thought process, they have been incorrect. meanwhile, they claim that I am incorrect in my own assessments, yet every action of theirs continues to support that I am right about their mental state & thought process.
of course, that's not all-encompassing, and I don't claim to know every single little thing people like that are thinking, feeling, or doing. there are many moments where I have an inkling about what might be the case, but I don't tend to declare that until there's enough evidence that I feel fairly confident in it. and, oftentimes, I can sense a general vibe, even if I can't nail down the exact train of thought or route that is being taken. ofc, I could still be wrong. that's never an impossibility. but that seems very unlikely when it accurately predicts the next action. unlikely, but not out of the realm of possibility.
the key differences here are that
a) I have a high bar for evidence in general (which is also why I hate commenting on situations unless I know all of the facts and multiple perspectives),
b) I have been proven right several times whereas the other person continues to be wrong about me,
c) all of the evidence presented as reasons for why the other person thinks they are correct in their own faulty assessments are all conjectures based on common social scripts and human reactions that do not accurately describe me and how I am known to react,
d) I have had countless people tell me that I have seen/validated/understood them (emotionally) in ways that no one else ever has, so I at least have some external evidence that it's not all in my head lol,
e) the errors made in "misinterpretations" show a disconnect with themselves and a lack of self-awareness, crucially pertaining to lack of self awareness of their own poor discernment of others
f) I never claim to be 100% certain of... almost anything I say? there's always a possibility that I'm wrong, and I'm more than willing to change my opinions on things given new information. being better at discerning people's biases doesn't mean that I'm correct 100% of the time, just that I'm generally more accurate than most other people tend to be (in my decades of experience on this earth lol)
the ironic thing about the vast majority of deductive reasoning is that the principles you use to end up at a conclusion can very rarely be used as a litmus test, yet (some) people seem to think that everything not passing that "test" (i.e. can be applied perfectly and in non-contradicting ways 100% of the time) means that it's useless and wrong, even if the tool that you're using is still accurate 90% of the time. you use multiple tools and angles of deduction to arrive at most decisions you make in your life. the cumulative effort gives you an intersection point that is as high as you can possibly get (with the information you currently have)--much higher than any of those single tools on their own. and, hey, sometimes that's still not very high. but sometimes it is!! sometimes it's damn near 100%! but if you had dismissed half of the tools you used to get along the way, solely because they weren't applicable 100% of the time, you'd have gotten a way shittier probability of being correct in the end. this also describes some of my frustrations with a.....lot of online discourse in general, because people will use fringe cases as a way to say that something is inaccurate (and therefore should be discarded completely and never touched or talked about ever), when that is itself a really bad idea.
(again, I am using the current situation as a bit of an example, but this has happened to me over and over again for most of my life, so I am using they/them pronouns in order to denote that this is more broad and general of a discussion/phenomenon that doesn't exist solely within this one incidence)