Because Skype is voice interaction where you can judge based off of tone in context. You can detect emotions.
Chat is emotionless. It is only given subjective emotion by the reader. A self-fulfilling prophecy would lead you to a bias against those you "want" to have some sort of disorder. It's a video game, played in short 20-60 minute increments. Autism is not something that simply can be determined in such a short session, especially over emotionless chat. Perhaps you could diagnose it over a long series of games, but not one game.
Strike that. You can diagnose people. So can I. I can tell people their mothers are dead when in reality they could just be in the other room. You can do anything. It does not make your diagnosis valid. Remember that being certified in something does not mean diagnosis is valid. You can think that maybe they have a disorder, which is fine. Opinion entitlement is perfectly okay. But, bringing it up in game is just a passive aggressive move.
You can deny it under the guise of helping people, and perhaps you truly believe that, but it is passive aggressive. Rather than lashing back out against toxicity, which is what I assume this is for, you are sugar coating it by "helping them" by saying "Oh, you are autistic and this is how."
The fact that anonymity exists makes it so that the "personality" conveyed in this short 30 minute average session is rather artificial. At best, you'd be diagnosing a false persona.
Just hit the mute button and play lol.