"fundamentalist" is used as a slur by some people (c.f. https://www.urbandictionary.com/define.php?term=Fundamentali...). The slur assumes things that don't exist in the word itself, so it's not obvious the word incorporates them.
From https://en.wiktionary.org/wiki/fundamentalism :
> The tendency to reduce a religion to its most fundamental tenets, based on strict interpretation of core texts.
Using such a strong word to define your adversary's position is a way to discredit them.