Probably true for the dedicated problem solvers (of which Tao is one IMO). But I doubt there's ever been a better time to be a theory builder (more like Peter Scholze, or Grothendieck).
Some up with an idea and leave the system to check it 15 different ways, and see whether you can simplify an existing body of theory. It'd be like having an army of lightning-fast grad students.
This is coming next. There's nothing particularly special about theory building. Successful theory building is always oriented towards solving a problem, because otherwise even humans can easily spam out a bunch of nonsense. This is a real problem that the math community has experienced on multiple occasions pre-AI. I'd go further and say that theory building is irrelevant if it doesn't help solve problems people care about.
I'd even say Scholze is not a great example here. Most of the work he's known for is progression towards the Langlands program - which is very much a problem to be solved, and one I would imagine he'd not be thrilled for an AI to one-shot. I agree that it is somewhat 'up the chain', in the same way that software engineering has not immediately disappeared now that performing coding tasks is largely automatable.
But I also take issue with 'never been a better time' - e.g. is this really the greatest time to be a software engineer? Everyone has AI psychosis and feels like they're a couple of breakthroughs away from being unemployable. The same is even more true in math - we've gone from failing IMO problem 6 last year, to solving NS. The rate of change is formidable, it feels like there may not be many places to hide in a few years.