I doubt this instance is being used by enough people to get trolls using it. It’s just because of the registration policy, which is open as opposed to the rest of the instances.
They just are too lazy to moderate it. Nothing to do with actual illegal content or imaginary trolls.
It didn’t change the repo. F-droid has always slower updates, because they’re compiling every app themselves. Izzy is release directly from app developer, so it’s newer. I’d stick to F-droid, so app dev can’t push anything malicious in the official app build.