Oh I'm so sorry, I was not aware medieval Europe had healing magic that influenced their research practices and their medical knowledge!
And also I had to miss the part where Divinity in Rellas explicitly forbids section of dead human body with a blanket ban! You know, just like Church in medieval Europe did, stalling the process of verifying medical knowledge.
I also had to completely miss the part where Rellas' scientific society is straggled by thousand-years-long worship of authority of just few chosen sources – just like in medieval Europe up to 12th century every new idea had to be rephrased and contorted until it sounded like merely commenting on teachings of either Bible, Plato or Aristotle.
You are absolutely correct, there's absolutely no explanation why Rellas' anatomy knowledge should be different in any way from medieval Europe's!
Even though economy, political dynamics and beaurocracy of Rellas are not really similar to "medieval", neither early or late, and instead look much more like variation on beginnings of early modern period.
And the fact that This Kingdom Will Not Kill Me is very consciously a "historical" romantasy isekai, and genre norm is that "historical" means "mixing elements from European 8th to 19th century, with antiquity and 1910s allowed in special cases", is absolutely not a valid reason!
You are, however, unambiguously wrong about two things.
First, if you refer to Galen's theory then humours are four, not three. Blood, phlegm, yellow bile and black bile.
And second, soap in most of medieval Europe was used to wash clothes, not people. Soap delicate enough to be used on human skin was only developed around 12th century, so rather late in "medieval". Soap and luxury scented soap are two quite different things. Lack or unavailability of soap is popular trope in romantasy isekai; if it was meant to represent medieval state of soap usage, there would be much more weight on other methods of cleaning.
Summing it up: yes, please go on and keep insulting authors both for your lack of understanding what genre were you reading and for your interpretation of "vague medieval". With such precise definition of medieval, I suppose Quebec also qualifies? It has intact defense walls after all.