Why would you imagine that? A namesake surname in a story set a century later is presumably meant to refer to a descendant. Especially for Sulu, the one TOS lead character that was canonically established to have procreated.
I mean, even stipulating that Sulu could live that long, what are the odds that he's somehow still a captain nearly a century later? It just makes no sense.
Besides, that reference was in an episode written by Michael Piller while he was working closely with Jeri Taylor. And Taylor's novel called it Hiromi Sulu, Demora's son. It seems reasonable that that reflects the original intention.