Someone on Reddit recently pointed out that a lot of this is mirroring the events in '93/'94, when TSR tried to claim everything concerning D&D online was theirs, and pointed to the old
rec.games.frp.dnd from the UseNet days. The flame wars were legendary. To my amusement, I noted that one of the contributors to that thread was Ray Winninger.
"Compatible with the world's most litigious roleplaying game"
edit: Was the following written 30 years ago, or 3 days ago?
"
Terms like Armor Class, Round, Turn etc. are completly save. As far as I know <owners of DnD IP> has got some kind of legal claim on the term "saving throw", but I do not remember the particulars. Remember, game systems are not copyrightable. They might be patentable (Parker has a patent on Monopoly, as far as I know), but no RPG has been patented yet. Given the state of the hobby, with any number of commercial and free systems sharing any number of mechanics it would be very hard to defend the patent against "prior art" claims."