Except he's not wearing the Arbiter armor that every single other Arbiter has worn, He's wearing Thel's armor from Halo 5 (is that the retconned Arbiter armor now, then?). Not that one should expect artistic consistency from 343i at this point, but when 99.9% of the character draws from Thel, it's a bit disingenuous to feign shock at the fact that some people assume that it is indeed Thel.
His blade is uniquely Thel's, from what we've seen.
His armor is uniquely Thel's, from what we've seen.
His level is uniquely Thel's, from what we've seen.
His catchphrase is uniquely Thel's, from what we've seen.
What obvious detail have I overlooked that would clarify the fact that it isn't him?

: to make things easier
: from an "explainability" perspective, while obviously leveraging
: certain visuals and audio/character cues to foster familiarity from a
: marketing perspective.

It's a shame that "marketing" overrides things like "consistency" or "creativity", but game development is a business first, art distant second (if that). The rub is when developers try to mask the fact with "we meant it all along" communication (in lieu of actual communication).

Every project has its own needs and
: characteristics, and we try and create the best scenario to fit a wide
: range of factors.

And this is perfectly understandable. Crap happens, demands and deadlines and all that. But things should be communicated if you don't want confusion. It's a bit odd to get annoyed at the fact that people make incorrect assumptions when you've done pretty much nothing to prevent this. What would it have hurt to inform players that an Arbiter who isn't Thel was going to be in Killer Instinct after the teaser was revealed. A short article would have avoided so much trouble (not that it's a big issue, like I said, the character looks neat, the level is great, the real problem is communication). I don't get why developers want to be so darn secretive, and still take issue with the fact that inferences are made due to the lack of information.
What else do you expect? Was the game plan really to run around after the fact and correct people rather than informing them beforehand? Did we learn nothing from the whole "Storm Covenant" debacle?

: I'll never understand why folks just seem to HAVE to find something wrong
: with things. Really sucks the fun out of so much these days.

I think you're reading a bit much into things. I don't see anyone saying that not having Thel or Keith David is a bad thing. The issue has been no information as to why this is, and every explanation is coming off as a familiar excuse. Communication is the big issue here, more than anything else. I'm no fan of this Killer Instinct game, but from what I've seen, the "not-any-specific Arbiter" looks like it'll be a great addition. Is it really so important to convince folks that it's not Thel? Isn't that pedantic "unsatisfyible bitching for bitchings sake"? Nobody's been complaining about the way that it doesn't make sense for the Arbiter to be fighting Ice Aliens or Sentient fireballs...