Yeah, I definitely wanted to leave it open ended. I had a solution baked into an earlier draft to cover "What if they just try to fight their way through everything." However, I ended up cutting it to save page space and because I didn't want to paint the GM into a corner.
My thought is that if they refuse to give up their magical items, they'll be confronted by a lone Man in a Golden Mask. If it does come to combat, then it does. It doesn't necessarily have to indicate everything afterwards will be combat. In retrospect, I think I could have done a better job of giving an idea of the number of people who are stuck here, which I picture to be at least several dozen, but probably not quite 100. There is a consistent enough influx to provide some plausible deniability that they weren't the ones who did it. From there, they know that if they flaunt magical items, they'll be confronted. If they can keep it on the downlow, they may be able to get away with killing the man on The Landing and then moving on through the adventure however they like.
It definitely has the potential to turn into a bloodbath if the players just want to fight everyone outright, but I think that lots of adventures contain that possibility for the right group. It could be an early combat that tells them a rule about this place that now they can plan around. It could be that they choose to give up their weapons and then steal them back from the giant treasure pit that they get tossed into while everyone else is asleep. It could be that they just choose to let them go and get them back before they leave.
I think one thing that I think might help with avoiding some of the more dramatic results would be if I rewrite the three things they learn to prioritize them learning that there is no teleportation out of here and that if they cause trouble, they're getting tossed into the acid. If they know that before being confronted about having to give up their magical items, I think it'll inspire more caution.