I think it's a combination of both...

Before AI people would still try asking questions on SO despite the hostility because it was the only/main place to get answers. Over this time, SO popularity was deteriorating.

After AI people who had bad experiences with asking questions on SO would ask the AI instead. This is because the AI wouldn't mark your question as a duplicate, wouldn't question its validity [1], etc. It would just answer your question. You could even ask it follow-up questions if the answer wasn't quite what you were looking for. All of this in real time, not waiting to get an answer.

[1] Unless the question triggered the safety gates that the AI companies have in place.