The places where I've seen unequivocal wins with AI are repetitive tech debt tasks that apply the same transformation across a large amount of code or refactor under a pre-existing test suite with good coverage.

I agree with this, but I'd add the caveat you have to do your own digging and researching (as the article says) to identify these tasks well. If you ask it for ideas, it gets painful.

YMMV (and I am very open to getting better at this).