I'm not a lawyer. But isn't the reason they had to go to reddit to get permission is because users hand over over ownership to reddit the moment you post. And since there's no such clause on Lemmy, they'd have to ask the actual authors of the comments for permission instead?
Mind you, I understand there's no technical limitation that prevents bots from harvesting the data, I'm talking about the legality. After all, public does not equate public domain.