The word "diagnosis" is associated most strongly with illness. If you say to someone "I've been diagnosed with....." Leaving it as a cliff hanger, they are going to be thinking "what bad thing have they been diagnosed with?" You can discuss the other meanings of "diagnosis" but that is what almost everyone would think, and in doing so they would naturally attach the notion of negativity to whatever you say you have been diagnosed with. "I've been diagnosed with bananas"; they will think that it is some kind of disease which sounds like bananas but is a bit different and they haven't heard you pronounce it properly. First they will get you to repeat it; "what?", then after you repeat it they will ask "what's that?" with a concerned look on their face. Try it out.
They know that diagnosis is negative; they are using it because they are calling autism something negative.
I am autistic and I do not think that autism is negative, some autistics do but there are many who don't. Therefore I do not think that diagnosis is appropriate. I think that an autistic person could be diagnosed with many negative things which commonly go hand in hand with autism, such as social anxiety, but I do not think that an autistic person should be "diagnosed" with what they are, aka to be diagnosed with autism. To be autistic is such an influential factor in so many areas of your personality, of how you think, how your mind works and who you are that to be diagnosed with autism feels like being diagnosed as yourself, which due to the displayed negative implication is not a correct thing for people to do to others. There is also such a range of relatively (to how one tries to function) positive and negative effects and/or associated neurological capabilities and deviations that autism cannot be strictly defined as negative, which the term "diagnosis" is implying, again as displayed above. Just my personal opinion.