.
There is some sympathy to be had for him.
There's at least two sides/slices/sides of the coin of Dave in all this.
On one side, there is Arneson the creator who from a very early part of the game lifespan was cast as the lessor creator, perhaps one who didn't really contribute that much to begin with or that his contributions weren't really that special and perhaps ought only be considered co-creator at the footnote/technicality level. That just plain stinks, and perhaps, worse, is most likely predominantly based not on the contribution itself, but what he vs. Gary did well past the point of that contribution happening. I can't find it now, and it might be apocryphal (and whether it is real doesn't matter for the illustration), but I remember hearing of an interview with Paul Simon where the interviewer kinda asked if he resented having to share credit on his early works with the clear unequal partner Art Garfunkel and Simon saying, in effect, 'you're mistaken, we both wrote those songs, and his later career-non-flourishing doesn't retroactively negate his contribution.' Arneson definitely could have used someone saying that earlier on, but the lawsuit and his alienating the rest of the witnesses* with his workplace issues early on and Gygax's own tendency to impulsively grab credit when possible made that non-feasible. This part is vaguely tragic.
The other main side is the Arneson the creator that thought he was in the league of the business persons needed to run D&D as it would be when purchased by 90s hot-off-MtG's-breakout-success Wizards of the Coast (and then Hasbro). This side, in combination with the letter, is more comic to tragicomic.
It's interesting that Peter turned down Gary as well?
Fundamentally, I think that's the larger point. Arneson's amateurish letter
highlights that he wasn't ready for D&D the polished professional corporate project, but it certainly isn't the dominant reason, and the real reasons were ones he shared with Gary. They both still were reacting to the game and audience and such from far in the past at that point.
At that point, it's not clear that he had something specific* to contribute to the game. And in that he definitely is right there alongside Gygax. Be it Arneson's The First Fantasy Campaign or the Goodman Games Blackmoor material or Gygax's plans for 2nd edition (Montebank class, etc.) or I think at some point he was asked what he'd add if he had the game still and he talked about 10th level spells. Both men continued to have ideas, but nothing that would make them naturals for running D&D for WotC at that point, nor even the go-to persons to ask about where the game needed to go.
*other than that which anyone has to offer which can be achieved through hard work and diligence and learning and listening to the audience
But it also speaks to why I don't come down too harshly on Arneson for this letter. I'm a manager in a large, faceless corporation. Every once in a while when looking to fill a Req someone passes along what we've taken to calling 'unclear on scale'-mismatches -- be it someone finishing up their bachelor's applying for a senior programmer position or someone trying to jump from analyst 2 to a principle position or the like. It's usually painfully clear that they misunderstood exactly what the position they were applying for entailed. Arneson's letter reads like this to me, and that I find really human and kind of relatable (not that I think they should have hired him for that role, just that I'm not going to be overly critical of him making the mistake, especially because of my next point).
It's worth realizing that it might not have been as obvious at the time (as it is in hindsight) exactly what level of rigor and professionalism WotC was going to be implementing with D&D when they acquired it (especially if Arneson didn't know how much they paid to get it). D&D had been 'dying' or 'so over' or 'that kids game you play before moving on to GURPS or Vampire' for a decade or so at that point. It's possible Arneson thought they just wanted someone to come in and make a not-lose-money product. We all know that instead they went with the rather exhaustive 3e push, but I can't tell you exactly when that became publicly communicated.
IME, that's not a salesman thing, that's the mark of someone who doesn't feel their intelligence has been properly recognized over the years.
I think it's fair to say that, in another world, Gary would have made a great college professor, which I think would have suited him better, but circumstances meant that, until he co-created D&D, his world was much smaller.
As others have mentioned, he was also emulating the Appendix N prose to which he aspired. That said, there are lots of things likely going on in Gary's word choice. He was a nerd before the modern nerd was fully conceived, and likewise shared a lot of traits we (well, most of us I assume) find familiar. There's using big words because we treated learning as a big deal so expanding your vocabulary is natural. And then there's the 'use big words to try to sound smart' component. Gary certainly had a vein of that in there (including the 'even when it hinders, rather than enhances, communication'). Beyond any of that, Gygax was choosing a style at a point when there were no rules about how to go about making a TTRPG book, so on some level, he was just fumbling about for an authorial voice with no stars to guide him. I do agree with their world being smaller, and I think both men are rather defined (fortunately and unfortunately) by being small-town/small-time participants of the established-but-niche hobby of wargames being thrust very unprepared into the role of figureheads to an entirely new pastime.