Everyone assumed the correct limit was just Newton's gravity, but it's abundantly clear that can't be right what with gravitational waves existing.

I think the author's, and other's points over the last decade or so has been "Newton neglects too much in the weak field" and that neglected bit isn't insignificant. The surprise here is that sticking that little bit -- which is needed for gravitational waves and in general the finite speed of gravitation interactions to occur -- appears to eliminate the need for dark matter in galaxy rotation curves.

Most surprising is that no one questioned the Newtonian limit being used despite weak field GR and Newton's laws predicting different phenomena.

The real test will be if CMB and other precision observations also work w/o dark matter.

If it’s true, or even if in this instance it’s not, it makes you wonder how many things there are that someone could just figure out with just the currently available information.

I think probably most astrophysicists did and realized GR does not matter that much at galactic scales. See my top level comment to this article for an explanation (disclaimer: not a professional physicist, I could very well be wrong).