It's not actually a quantum theory, just a quantum approach. We use simplifying computational models for both black holes and large (or high energy, or just wei

It's not actually a quantum theory, just a quantum approach. We use simplifying computational models for both black holes and large (or high energy, or just weird) quantum systems, and these researchers think they found success porting over some of those techniques. We really, really want spacetime to behave like quantum fields do, but in general it just behaves completely differently. In this particular instance though, those differences don't matter enough to make the tools of QFT totally useless. Quantum physicists spend a lot of time integrating lots and lots of tiny effects that may or may not interact with each other in strange ways, which is handy experience for working with colliding black holes. Or rather, solving a subproblem coming from certain simplifying techniques.