Not because he knows that the advance is unwanted (though it could be that he knows, it just depends by case), but because he *assumes* any advances *from him* will be unwanted. Getting rejected every single time would make it hard to assume "Ok but surely next time will be ok", and so you would operate, from the start, under the assumption that the other person doesn't want to talk to you.

At a certain point it doesn't matter if he knows or assumes. The end result is internally he on some level thinks the advance is unwanted. If I didn't know, but I assumed someone didn't want to date me I wouldn't ask them out. If you're operating "under the assumption that the other person doesn't want to talk to you" why are you wanting to date them? I only want to date people that want to at least talk to me. That's part of what contributes to my assessment it is a nice guy scenario. A legitimately nice person who believed someone didn't want to talk to/go out with them would not still ask them out just because they can add a "sorry" beforehand.

I know and have dated a lot of socially anxious people, and in my experience the apology is generally nice guy behavior. Maybe it ends up being a feedback loop of social anxiety>low social skills>less likely to get a date>resentment builds>lashing out, so social anxiety tends to be more associated with nice guy behavior, but I think social anxiety isn't the underlying factor here and it's actually low social skills more than social anxiety. I generally do not see social anxiety manifest in an apology like that. I'm sure some actual nice people have opened with apologies, and obviously I recognize it's a social skill issue, but that doesn't really change my read of it or my personal experience. Again, totally possible to apologize and not be rude after, but that's generally not been my experience. If they're apologizing for the act of reaching out in my experience it's generally that the first message is rude or there will be rude follow up messages.

I can't speak to your experience, but it seems like even you say you would be more likely to express your social anxiety in an "opt out" option. I think that's a level of social skill above the apology, and that level may contribute to you knowing that being rude is not going to help yourself. Maybe it's just that people without the social skill to go the "opt out" route instead of the apology route are more likely to be unable to handle social rejection and more likely to fall into antisocial behavior. Just speaking from my experience and the experience of the collective of women I'm in community with.