As above, they check at the boat dock and bus stop before you board. If you’re not staying there, you’d need an ADR to board the boat. And once you disembark at FW, you’d only be allowed as far as the Settlement. Guests arriving by car are turned away if they’re not staying there.
Yes, they’ve done this before, to keep the campgrounds from being overwhelmed by visitors at special times like Halloween.
ETA - I didn’t have time for an extensive search, but this from 2021 is an example of the restrictions.
Post in thread 'Halloween night - resort guests only?'
https://www.disboards.com/threads/halloween-night-resort-guests-only.3857467/post-63484047