The funny thing is that this is actual flat earth lore. And no, you can't just go there and see it, because the government won't let you, you'll die, you just turn around and just forgot what you saw. Depending on your local flat earthers.
I really like how similar to religion it is. Yeah, when you die you go to heaven, you juat have to trust me bro.
Flat earthers are so adamant about their beliefs BECAUSE of religion. They believe the earth is the center of the universe because why else would god put humans here, so they need to justify that with stupid, verifiably false claims.
The truthers: they legitimately believe in the flat earth. They generally haven't traveled much or at all and have no idea the scale of things. They aren't willing to test stuff themselves, but they will parrot anything that sounds decent.
The grifters: much like telemarket scammers asking about something antiquated (like TV boxes) they know their audience is likely going to fall for whatever they have to offer. They make videos online constantly, ask for people to join their group, squeeze whatever they can from people who want the echo chamber.
The "anti" 's who have become the Qanons: the flat earth isn't the whole iceberg, it's barely even the tip. It's all just a method of control that 'they' want you to believe in so that you are simple sheep to their new world order and anti religious rhetoric. They don't really care if the flat earth is real, it's just close enough to their beliefs that they'll assimilate it to spite 'the man'.
And the fourth group that aren't flat earthers at all: the trolls. They fan the flames, make the images that don't fit the models, and generally seem to ally themselves with the flat earthers, just to laugh about it later, while also getting people who notice stuff like this to so laugh along. Flat earth is the joke and their assistance is the punchline.
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.
I haven't seen it, but it wouldn't shock me if they saw a clearly fictional TV show (I doubt they're bright enough to read a manga) and decided it held some great secret truth.