Are refs allowed to annul goals?
Anyway, is the game actually going to be replayed?
Yes, they are allowed to do so.
Whether the match is replayed isn't 100 % certain, but Hoffenheim has filed a complaint and the DFB is not known to go against precedent decisions + they would earn a shitstorm if they'd let this slide. So I'm pretty sure that this match will be replayed.
Edit: I just re-read the DFB statutes which state that the final decision is made by the FIFA on whether a match is replayed or not, but the DFB present the case. Since the FIFA hates replaying matches, there's a risk factor here. Time for the DFB to show some balls.