They do calculations using a theory where gravitons are assumed to exist, and then their results are valid in that theory.
This kind of work is critical to refining theories and highlighting/fill gaps in knowledge.
"It was generally assumed within the scientific community that resolving this paradox would require a huge paradigm shift in physics, forcing the potential reformulation of either quantum mechanics or general relativity.
"What we found—and I think is particularly exciting—is that this isn't necessary."