It won't stop them but it severely hinders them.
Beehaw's registration process takes time. Rather than spin up a bunch of accounts instantly and troll rapidly (causing massive admin/moderator overhead), they'll have to register direct with beehaw. This limits the number of accounts action has to be taken against.
Looking at it from an attackers perspective, is it worth it to wait 1+ days of going through a registration that requires the user submit answers to a questionnaire, just to troll users on beehaw? Probably not.
Until other instances defederate, your path of least resistance is to stay put. When that changes you'd reassess, and move elsewhere.
I understand that. Beehaw has a questionnaire they require to be filled out as part of their application process.
You can filter verbatim responses (if 100 accounts have the exact same responses, they're probably not legit) and do other things to help cut down on bot accounts being used for trolling. This is why I said it's a severe hindrance, not a 100% perfect solution.