From what I read and maybe my own on-the-spectrum-traits the ability to focus intensely and for months on something. It really helps for studies.

This is a myth. Authism is a spectrum disorder and you don't meet the more disfunctional cases very often. A few percent are also savants but not everyone in the spectrum has some superpower. The severe cases are very crippling and place a heavy burden in the family (specially because the life expectancy is normal so you will not be always available to care for your autistic son).

I think the discussion should not be whether autistic people have some kind of superpower but whether there are parts of the autistic spectrum that can't be thought of as a disease.

There are obvious examples of Autism that are absolutely objectively terrible for the person experiencing it, this is not about those cases but about the "less severe cases" where an autistic child can go to a normal school and lead a generally normal life. If that is the case, I think the line between what you call a disease and mere personality traits is actually not very clear. Instinctively I would be very careful to describe someone's personality as a disease just because it is different from other peoples behaviour.

The problem with my thinking on the other hand is probably that the definition of the word autism itself might be unclear when considering these edge cases.

It's not a myth. I'm convinced that, all else being equal, being able to obsessively focus on one thing will bear its fruits, even if it can be disastrous in areas like social life and personal health.

The idea that people on the spectrum are more likely to be savants is a myth (the opposite, that savants are more likely to be on the spectrum less so, from what I understand), but the person you responded to made no such claim.