The reasoning was its only a debut for the purposes of the award if its not based on an existing video game franchise.
Since Hearthstone is based on Warcraft, it's disqualified. But since Middle Earth was based on (obviously) Tolkien lore, rather than any pre-existing Tolkien inspired video game, it's eligible.
So yeah, the reasoning is technically sound, though I will say kind of flawed if you think about it in the meta sense; Warcraft as a franchise is heavily based on Tolkien (as most Western fantasy is), but somehow the gamebased on the Tolkien IP is considered more debut-esque.
Honestly I think the "Best Debut" category needs to go altogether. What purpose does it honestly serve the awards at this point? Every time they've done it, they've awarded it to the game they go on to give Game of the Year to anyway (TLOU, Shadow of Mordor), so you have to question the value in spending time giving what is basically a GOTY award with an asterisk.