Cutest member of the party.Bear girl is 100% a waifu
Tubs have been reasonably common throughout history. Which is what he had there. A simple wooden tub, which you'd easily find in households that had the spare space (which wasn't always a given, especially in cities and towns). And aside from that, bathhouses were also common in many cultures.Even poor folk have taken a bath in rivers and such. And the creation of bath itself is not a complex thing.
Every larger medieval settlement had public baths, which also served a social purpose. They might not have been as extensive as Roman ones, but good enough to make bathing in rivers only a thing for travelers or the utterly destitute.This world doesn't know about cloth put in warm water nor baths? There is a limit to how stupid things can get. I know this is a trope. But they are world with lifestyle magic for god's sake!
The mangaka could have had the girls praise him for putting in the effort and thinking of making the bath in this kind of way, but that they have never had a bath? The A class adventurers? Even poor folk have taken a bath in rivers and such. And the creation of bath itself is not a complex thing. It just takes time and effort (depending on if they knew the tricks for simple baths) hence why poor farmer family might not do it often.
Seriously, if Romans could do warmed baths easily, AND public ones, there is no bloody excuse here for Mr fantasy world.