The idea that D&D is also the only game that defines its genre is false. There are many tabletop games which are similar to D&D that are not referred to as, D&D such as Dark Heresy, Warhammer 40k, Halo Mythic, Only War, Call of Cthulhu and many more.
My friend’s game is essentially a role playing game, but he felt like most role playing games have a negative stigma. D&D stands apart because of its complexity and the math involved.
By calling his game D&D, he is trying to avoid the cringy stereotype of role play. My friend’s strongest reason for referring to his game as D&D is that by doing so, he is avoiding confusion among people who are unfamiliar with the original D&D. His point is that if the game is referred to as D&D and somebody doesn't know what that means, it's a lot easier to just explain his game than it is to give the game a unique name and be forced to explain it is a D&D-esque game with it’s own characters and rules. Essentially, why bother taking the time to explain the similarities and differences.
While it's good to keep things simple, it is important to distinguish the two games for the same exact reason. If both games are named D&D then whenever someone decided to play the original D&D there is going to be a communication problem. Not to mention that D&D is copyrighted.