That's interesting - I always assumed since Thanos is an Eternal and therefore an offshoot of humanity, that he (and any other Eternal) would also be eligible to carry an X-gene. There just aren't any mutant Eternals because their birth rate is so slow. But since Thanos has been described as a kind of "mutant," I didn't figure it was much of a stretch to describe him as a mutant, since it makes sense according to Marvel science that he COULD be.