The only reason D&D is so big is that the profit lets it advertise and have a presence, so more people know about it. That is a benifit to the genre and encourages creativity.
The idea that D&D has hoovered up so much of the world as to deny people of their own creative expression is absurd and I couldn't get past the first paragraph without thinking the author is off his rocker.
While other TTRPG's are still free to exist, it is much harder to convince a new player to hop on board with anything else (or to even explain to them how there are games very similar to Dungeons and Dragons that aren't Dungeons and Dragons).
It doesn't help that things like the term "DM" are patented by WotC, too.
Play was all DIY and the saddle bound Guides were ambiguous suggestions and a gateway into the world of more-than-six-sided dice.
Advanced D&D was a different game sold at the Dalton's in your local mall. Each of the ever expanding line of hardcover rule books - that game had rules not guides - cost about double the ten bucks of the old three guide boxed set (Chanmail was another ten bucks).
My understanding from the article is that the book covers that transition or succession or whatever from guidelines to rules. From a stapled 'zine to manufactured product lines.
Is that the death of art? As such reports tend to be greatly exaggerated, probably not. But there was a change. The game got canon.
And cannon canon https://www.dandwiki.com/wiki/Cannon
The history of D&D is an example of capitalism commoditizing creativity.
Initially, there were some very thin rules about character stuff to accompany miniature wargaming. This was not terribly interesting to most people. Wargamers or otherwise.
That grew into some still thin and permissive rules and more of a focus on playing a role and storytelling. The onus was on the players to do most of this. There wasn't much in the way of example or instruction. This attracted the crowd we think of today as makers - self-directed problem solvers who like to greenfield.
That all seemed sort of interesting but not approachable enough, so, in typical capitalist fashion, there were more and more manuals and rule sets and published scenarios. There was a lot more hand-holding for the creatively needy.
Meanwhile, simple algorithmic dungeon crawlers were created on computers. There was only a thin whiff of role playing as theme in combat-mechanic- and metric-heavy CRPGs.
Then we bounce back to TTRPGs who've learned from CRPGs and they become something a bit like CRPGs with slightly heavier theme. Almost all the creativity is sucked out of it. What we used to call "munchkining" prevails. The only meaningful decisions are in character creation, itself a strictly bounded system.
How did this happen? At every step along the way decisions were driven by profitability, economies of scale, and the general commoditization of creativity, turning every game into the same pleasant McAdventure.
License here: https://www.d20srd.org/ogl.htm
Basic rules, classes, monsters etc are included: https://www.d20srd.org/index.htm