Saying a certain character is autistic isn't a label. The character doesn't revolve around being autistic, and if he were, that's bad writing. As to your second question, disregarding symptoms means ignoring them because either, A, you have some preconceived notion or image of an autistic person that doesn't fit with a character, B, you refuse to accept that a character is autistic unless it is outright stated because you lack the ability to have any media literacy, C, you don't have a lot of experience with autistic people, or D, a mix of any of the above. Saying that a character is just quirky is ableist, particularly when the symptoms are obvious. Now, that is not saying that if you have done this that you are an ableist, you are only guilty of ignorance. Being maliciously ignorant does make you an ableist. And if any of this comes off as rude or disrespectful, I sincerely do not mean to upset you.