The D&D world is not covered by the OGL. its the core mechanics that have been mirrored by not just other TTRPGs, but video game RPGs as well.

This is like if the names of the functions in the c++ STL (and their argument/return types) was released under gpl, and they came out and "updated" their license to be basically cc-by-sa-nc but worse and also claimed it retroactively applied to the license. It could technically have repercussions to other languages that mirrored the style.

Now the fun question! how much of what was covered by the OGL was actually copyrightable?

Ideas and facts are not copyrightable, arrangement of ideas and facts can be copyrightable with a higher bar of 'creativity', but the barrer to change before it becomes a new 'arrangement' and not covered is also lower.