I’m not autistic but multiple times throughout my childhood a few professionals thought I had ASD and/or ADHD even though in the past it’d been ruled out because my behaviour was caused by the disorder I was diagnosed with. Misdiagnosing does happen a lot in the system and in my own case they thought it was more likely to be ASD/ADHD because of a gender bias but it didn’t take them long after sitting down with me to confirm that I had what I was diagnosed with even though the other gender was more likely to have it. Professionals can be the worst when you’re diagnosed so I definitely understand where those seeking a diagnosis come from when they say things like this but it doesn’t excuse it at all and it definitely doesn’t make self diagnosing valid. If professionals still argue over cases like mine (been in system 15 years) then people definitely can’t self diagnose themselves and be 100% sure.