Game rules/mechanics usually can't be copyrighted other than just the literal text of descriptions of them. Does this give anything you couldn't have by cloning the rules with different wording flair?

The thing is, compatibility with D&D wasn't necessarily about mechanics. Fundamentally TTRPG mechanics are really just about simulating probability distributions with dice. There's the broader question of how to use a given probability distribution where.

What D&D editions do is develop a set of base mechanics, basic probability distributions, and then create a framework on how to apply them. This includes monsters, races, and classes which have particular attributes or play feel. This includes common roleplaying conflicts and guidance on how to adjudicate them. What a lot of indie RPGs that used OGL 1.0/a did is they made references to things from D&D and used them in ways inspired by D&D. Think "elves" or "faeries" being associated with the mechanics and tropes you'd expect. Now with the new CC-BY-SA 5.1 SRD, you can make explicit references to Elves in derivative works and also license it under CC-BY-SA.

There were always completely separate systems that borrowed nothing from the play feel/world of D&D that had nothing to fear from this change. Those communities/creators had nothing to fear.

Ah thanks, I had thought the core thing was content agnostic.

"can't be copyrighted" may be true in the ideal, but there's plenty of haziness around the edges. The particular coloring of each type of dragon is mechanically unimportant, but is a nice mnemonic for the type of breath weapon damage they do. Is dragon coloring function or flavor? If you're a small publisher, you don't have the lawyers to compete with Hasbro in court long enough to find out.