A large subset of flat earthers believe the world is surrounded by ice walls that THEY don't want you to know about and won't let you see. The bathtub is just representing that container of ice.
Because demonstrating solidity needs to happen, obviously.
I'm pretty sure this is trolling though. I refuse to believe the Ice Wall Mega Earth subset is real.
My favorite scene is when the 2 flat earthers visit NASA. They are in a simulator making fun of it cause it won't start for them, and as they walk away the camera zooms in on a giant "start" button they both managed to miss.