Its helpful if your intention is to play a Dungeons & Dragons game that features cosmic horror, not try to make a 5e version of Call of Cthulhu .
I think that's the point he's trying to make; the DMG rules for gritty or firearms still reflect the fundamental basis of the game; they don't replace or neuter classes, they don't remove spells or spellcasting, they don't make other elements of the game inoperable like removing Hit Dice. They play within the framework.
I think its worth cautioning that trying to turn D&D into something its not is a fools errand. I have considered in the past converting Masque of the Red Death from its 2e/3e roots to 5e; but the sheer amount of re-writing necessary to accommodate a low-magic world set in 1890's Earth is practically a new game unto itself. Just to start, you'd need to re-write every class (and cut most of them) and subclass, every background, large swaths of the combat rules and magic, and the entire equipment chapter right down to the copper piece. All to fit a square peg in a round hole. I'm sure there is a system that handles Gothic Horror much better than D&D (and I'm all ears if you have a suggestion for one) so making D&D try to handle Penny Dreadfuls is more effort than its worth.
Compare that to Curse of Strahd, which is basically Dracula/Transylvania in all but name, but doesn't attempt to emulate the gaslight world of Victorian London but instead sets the D&D tropes up front (even as it twists some of them). You are still an elven mage or a dwarf barbarian or a dragonborn paladin facing off against Strahd. Your magic might not work right and Strahd might have far more power than a common vampire should, but its still fundamentally D&D, just with a thick coat of Brahm Stoker on top of it.
So the point is that D&D is best when its D&D with flavoring or seasoning, not when its contorted to emulate a genre its ill-suited to play in. D&D is not a generic RPG like GURPS is; it excels best when it does what it does.