Therapists make comments like that all the time. More (maybe most) of them need training in patholigization and how and why its harmful. Ive dealt with therapists saying to me "oh damn that behavior is not ok! Sounds like shes bpd or something", and like i can totally see someone who wants to "fix" you latching on to comments like that and substituting them for an actual diagnosis. Not to mention even shittier therapists who actually try to dx someone based entirely off another persons reporting of them.
As far as countering the accusations, you could go the vulnerability route, and level with them and explain adhd, autism, and your direct experience of autistic burnout, laying out how it affects you and what you do to mitigate it. You could also try telling them that the way they approached you came off as aggressive? Idk, tbh when people have done this stuff to me theyve either been like 18 and literally dont know what theyre talking about, or older and its kinda been the lifting of the veil that makes me distance myself from them.
Absolutely not on you to do the labour of educating them. You could just be straightforward to the point and tell them they dont know what theyre talking about, youve got a good understanding of yourself, and are doing what you need in order to take care of yourself. And yeah they should be talking in therapy about themself, not trying to pin problems on you or do armchair dx stuff.