Why would they use a potion if it's "basically poison"? I swear these isekais just love having the inhabitants of these worlds as being less than brain dead.
Let me guess, they also don't have soap, their cooking is bad, and they're shocked that horse feed can also be eaten by humans (rice).
Wow, you guess most of Cliches here but allow me to correct. I'm not defending the Author though, I quit reading this Novel at a certain point.
Author's cook:
In the Ancient time, potions still tastes normal,
war happens,
Alchemists die,
potions' manufacturing method lost,
no more normal potions made,
After centuries of research, new Alchemists made the "gruesome potion", that can heal injury but posion you instead
King something-sama wants to make a Potion Revolution to make "normal potion"
They succeed making "normal potion" and let everyone drinks it
Then Plague kill 40% of population
They can't find the reasons
King something-sama, all Alchemist Researchers became Witches and were killed
potions' manufacturing method lost once again
"gruesome potion" becomes standard operating procedure.
The Royalties also banned people from changing "standard operating procedure" or "it's treason then"
TL;DR: They don't have Soap, their cooking is delicious so is MC's cooking, Rice came from the East and eat normal, etc.