There are reefs that have been around since before the dinosaurs roamed the planet. I was told that some of them are reefs in the farthest sense only but they are still ecosystems that are reef-like. And the planet has experienced a couple of warm periods with temperatures quite a bit higher than we have them now. Oh, and even the Great Barrier Reef has been around for about half a million years by now. And we know that during the last couple of warm periods over the last 10K years, temperatures were quite a bit higher than today for centuries. How did all the reefs survive in the past?
Even 1.5C would kill 99% of all coral, according to the study. But I think there is room for a little doubt.