For context, autism doesn’t really have anything to do with accuracy of maths. It has symptoms like difficulty communicating and special interests.

It’s hard to explain, but “your autism has failed you” just feels weird. I have autism myself. I’ll probably struggle to explain why, but here goes:

• The original comment was using autism as a joking explanation as to why they enjoy doing calculations. The reply turns that against them: “your autism should have made you good at this, but you failed.” It’s ok to have self deprecating humour, but it’s less funny to deprecate others.
• It reinforces the stereotype that all autistic purple must be good at calculations, logic, and detail oriented tasks. As if it’s a superpower or special talent, and this person’s autism powers simply failed to activate correctly. It turns the joke into a standard they’re supposed to meet.
• It makes the mistake seem more serious and personal than it is. We’re calculating condom based floatation devices here lol

They could have just supplied the corrections to the calculations without making it about their identity. I assume (in good faith) that they meant it as a joke as well; it just came across as a bit insensitive and demeaning.