As someone who has just heard that term and done exactly 20 minutes of skim-reading a Warhammer-based explanation of diegetic essentialism and why it's bad (this is the theory needed to bring about the revolution) couldn't the deckhead merely argue that the themes and aesthetic for this story (whichever the two proponents are whining about) are reliant on appearing self-consistent and its failure to do so undermines those two critical components of fictional narratives and thus undermining it as media as a whole. Obviously, this argument doesn't work for all media, but the type that would attract someone who is a deckhead likely could have them stretch some application of this argument for it. However, another caveat is this would very much depend on what aspect of lore and self-consistency is being argued about.
Edit: Actually, I found an example. As someone who spent too much internet time in rationalist adjacent internet spheres as a teen (the sphere the Zizzians originated from) something like HPMOR only conveys its themes when appearing internally consistent, (however ironically it often comes into contradiction with aesthetics due to this, and understanding that is not as comprehensive as needed for the scientific topics it covers leading to inconsistency). If it fails to appear consistent, it undermines its own themes and values and the conveying of them.
I'm being a bit tongue-in-cheek and don't personally buy the argument fully myself and I'm partially just killing time between meetings and got tired of arguing on
. The argument I was proposing wouldn't be though that all media is reliant on that consistency (and I agree with your points about attempts at portrayal) but rather the argument is the particular thematics and aesthetics of the work being argued about at that moment. I was using a piece of media in a particular sense as opposed to the general. It is actually why my edit mentions the rationalists and HPMOR as due to how rationalists' belief functions (from my admittedly hazy recollection) not attempting nigh-seamless consistency would undercut some of the rationalist messaging particularly about storytelling norms.
Obviously, this check fails against broad-spectrum storytelling if someone has seen the word Aesop before. I am also assuming someone who is being a deckhead and isn't just using it as a veil to prevent a change they don't personally like (which then I ask you why are you arguing with them) and isn't just yanking your chain to fuck with you, (I know, I'm really shifting goalposts here eliminating 90% of them) I presume they would try to either narrow their deck-headedness into a slimmer category to attempt to shore up their argument, especially since much of the consumerist identity culture mentioned in the Warhammer essay means they likely are not invested in whether their argument functions for art as a whole and only needs some form of it to hold for the things they like and are invested in.
If you want my actual personal thoughts on this, the entire ideology can basically just be undermined with this section of your comment: