Though I've spent a lot of time in Belize (further south, mostly) and I'm a marine biologist, sharks aren't my thing either. (/DISCLAIMER)Grateful that the crew handled this correctly and that the poor girl is stable. (DISCLAIMER!) I am not a shark or marine biology expert, but I tend to think this was caused by a bull or tiger shark versus a reef shark.
I've seen lemons as well in the vicinity of Half Moon Caye, so might add that to the list of possible suspects. A guy I knew decades ago got bit by a bull shark diving in the Blue Hole. It was back in the day of legal spearfishing there, and he had a dead fish with him....
I think it was about 4-5 years ago when the guides at HMC told us that blacktips would circle us when we first descended and decide if we were food or not (their words). They'd realize we weren't, and go on their way. That in fact did happen. (The circling, anyway. I'm not a shark mindreader.) I've dived there since and never saw a shark. I'm guessing they come and go. I've never thought they were dangerous, but the behavior I saw seemed consistent with the guides' story of what was going on.
It's worth noting that despite rapid helicopter evacuation, it's not a short trip to the ER from HMC. Where I teach most of my SCUBA students at home, an ambulance with sirens could get you to a level 1 trauma center in 10 minutes. More remote sites might take 20.