The idea that mutants have a shorter lifespan because their own powers eventually "burn them out" so to speak is actually a neat and logical idea on paper (although sort of a cross-between Todd MacFarlane's Spawn and JMS' Rising Stars). It also, potentially, makes Fabian Cortez the most dangerous mutant alive.
The problem is it doesn't really fit with what Claremont himself established in past continuity. One could reason Xavier was crippled by the Shadow King because Charles' own powers made him more susceptible to injury, but here states he doesn't know how it happened? And does he not remember Magneto was essentially given a whole new body after the Savage Land? And how does that even fit with someone like Wolverine, or Sabretooth, or heck, even Mystique--all of whom are supposed to have healing factors that prolong their lifespan? And I guess John Byrne's story about how Claremont didn't know Apocalypse is a mutant is also correct since, apparently, mutants cannot be immortal by definition. Same with Namor who I suppose because he's "half-Atlantian" somehow doesn't make him a mutant (even though technically it does as the whole Dark X-Men thing proves). And what about mutants who were well past their 60s like Destiny?
Guess this is what we call an example of Claremont saying "Since I made up the rules, I can bend them whenever and however I want to."