So I think what's happened here is that Elon is getting a lot of flack for the fact that killing the free API is going to kill loads of really good products, bots and research. So in order to try and drown out the complaints he's doing his normal routine announcing a feature that doesn't exist. But no, let's give him the benefit of the doubt, I'm absolutely sure that he's figured out how to track exactly which ads were placed in which locations, exactly what their rate is, annotate that back to users, and then pay the revenue out to them. I'm absolutely confident he's figured this out despite the fact that a large number of Twitter users purchase Blue through the Apple appstore and therefore Elon doesn't even have payment method via which he could actually pay out the money. "Starting today".
The fact he's done this probably points to how badly the API announcement has gone, rather than anything about users monetizing ads.