How is it possible for them to not know I'm different regardless of whether I mention autism or not?
For starters that's simply asking the wrong question. It's a job interview. Not a psychological evaluation. Whether you are "different" may likely be irrelevant to their hiring requirements, apart from discussing certain issues which may or may not put their business liability at risk.
You have a logical and mathematical mind. So consider the ratio of NTs to NDs. That an overwhelming number of them likely has never had any contact with neurodiverse persons, or has any incentive to learn or understand them. (I think the CDC has revised this ratio to 1 in 36.)
Such odds inherently favor their ignorance and even more,
their indifference. Neither of which will work for you if you expose your autism in such an interview. Not to mention it could be interpreted as a plea for special consideration that an interviewer/employer may not appreciate.
A competitive job interview is anything but an act of charity. It's a screening process where if you don't fit their mold in some particular way, they'll likely put your application on the bottom of the stack, or simply dump it in a wastebasket. The last thing you want to do is to throw something in the mix to diminish your chances of being hired. You simply want to present yourself as the best possible candidate- not one with special needs, excuses and exceptions. It's all about what they want- not what you want. That they owe us nothing.
Yes this sounds harsh, but you are entering a highly competitive field where you need to shine as a prospective employee. Trying to sell them on your autistic traits and behaviors could spell disaster. All compounded by a new administration intent on abolishing all forms of diversity, equity and inclusion in public sector workplaces. That alone should be incentive for you to keep your autism a secret in the American workplace at this particular time.
So think of more positive things, such as thoughtful questions to answer, and to project a certain enthusiasm about the job. It also helps to "underwrite" a prospective employer. Do some research to find out about them to ask those intelligent questions others may not even think of.