I agree with the rest of your post, but uh, it would be considered extremely gay if two Japanese guys stayed in the same hotel and shared a bath together.
No.
OK maybe worth elaborating. For same sex friends or co-workers to share a hotel room, is very common for most of the world. It's only gay for North Americans. It's standard practice for business trips in most countries, including most Asian countries (that don't have homosexuals equality), to book a double room for every 2 travelers. I only switch to single occupancy a few years ago because now I have the budget to afford it and that way I can watch porn and fap. It's nothing to do with being gay sharing a room with a co-worker or a student.
For taking a bath together without any sexual or romantic consideration, it's also not so out there for many countries. But it's more specific Japanese that it's a
thing. That's why almost every yakuza movie has a bath house scene. It might a slowly declining trends, but it's still current enough that you'd never hear such kind of gossip "I hear A and B shared a bath together... REALLY, are they gay???".
Opposite sex shared bath is now so passe that it is no longer considered platonic.