AI-powered answers may soon be part of Reddit's search

That's not strictly the reason. The surge in AI-generated content happened more recently than the uptick in users using "Site:reddit.com".

It actually goes back to Google's algorithmic updates favouring ads, and low-quality content from high authority sites, whilst purging a lot of question/answer-based content like Quora and StackOverflow. Users noticed and reacted accordingly.

I think Reddit are just trying to mush together two trends when they don't need to. Search users noticed that they got better Google results by using a parameter and limiting to Reddit. Reddit should take that win and at best run with it for a Google-like search landing page, sure. The AI stuff is at best buzzwords and hype for investors.