I stepped out because I figured continued argument was futile, but I did not step down.

The "Edax Test" is simply using your own arbitrary criteria for what constitutes a well-written antagonist, and trying to make it a hard and fast rule. In this case it's "is related to the myth arc," it seems. Zenos is wholly unrelated to that, so he is deemed "poorly written." (Given as Nidhogg also has nothing to do with the myth arc and him destroying us wouldn't advance said myth arc, does that make him poorly written as an antagonist? If not, how so?)

Zenos' narrative purpose is not to advance the myth arc. His narrative purpose is to serve as the PC's foil, and perhaps their shadow archetype (the former is tenable, the latter is dependent on whether or not you accept Zenos after his rant in the Royal Menagerie). He is not the most complex or interesting character, but that does not mean he is poorly written. Zenos is a sociopath, behaves pretty realistically for one in fiction, and has the writing to fit. What about him is poorly written, beyond his lack of relevance to the myth arc?

Everyone's a critic. The quality of a film (or whatever) is subjective, beyond production values. That's why review aggregator sites exist; they determine the approximate objective quality of a film by taking a large number of reviews, determining how many are positive and negative, and giving a score based on that. However that is only approximate; you could still end up disliking a film with a good score and loving one with a bad score. Arthouse films tend to do well with critics, but laypeople usually don't care for them; blockbuster films tend to do poorly with critics, while laypeople tend to like them. (And before anyone says "But Marvel...", other than the Avengers duology closing out Phase 3 I've found it quite pedantic as of late despite high marks from critics and audiences alike.) It's subjective.

Trying to pass off your own subjective criteria as to what constitutes a well-written antagonist as objective truth is just self-aggrandizement. I must ask: what is objectively bad about Zenos' writing? (And myth arc relevance != quality.) What he has captures his character quite well if not perfectly. What is lacking?