You mean like him saying Mirror Mirror is from an episode of Star Trek that has the same exact title? Like I said before, if you put your own spin on something you aren't violating a copyright issue.
Almost every story ever written is based on some story the author has read or heard about. There hasn't been a truly original story in probably thousands of years.
Actually, if Star Trek wanted to push, they might be able to stir up trouble, however the name itself isn't copywrite. Also why Jim has avoided quite a few DnD monsters despite most being under an open umbrella because one group
could later claim rights if they so choose. Now if They could viably prove Jim was taking from their revenue/base by putting out something of the exact same nature then yea, Look at fortnite being sued by PubG, they aren't the same, but that one little aspect was Battle royale enough to force Fortnite to shut down soon.
Star trek being sci-fi and a show it's kinda hard to say any such thing, but a book on fantasy where in Merlin archetypes are prevalent? Yea.... absolutely. Ya'll seem to forget this is America and they can choose to sue over the slightest provocation regardless of an assured victory. You seem to forget the guy who patented the motor vehicle and waited 20 years until they were being produced to press the patent, which caused ever single competitor to fold and give royalties except Henry Ford whose work helped overturn said patent.... After a lengthy legal battle that would in our case, basically kill the DF.
*compare it to mythology, compare it to works already in the public domain like Shakespeare. Stop saying it's a rip off of protected works before someone decides to take it seriously in the wrong way.