Roger seems to be the type that wouldn't bring it up unless explicitly asked if Luffy is any indication like how he responded to Ivankov's assumption regarding Ace's real father, or how none of the other Straw Hats knew about Ace or Garp. A simple "you didn't ask" I'm guessing would be the response.
Anyway, let's say Roger was a Power User who's ability didn't actually do much of anything in a fight. So he probably just fought with that sword/gun combination we saw in chapter 0, and I'm guessing a guy like him was easily capable of staying out of water. During his tenure as a pirate and gaining strength and infamy, it simply got to the point where Roger being a Power User wasn't even a possibility in other people's minds. Similar to how Brook was before the Yomi Fruit taking effect and subsequent return to his body's remains a year late.
Like let's say…the Hito Fruit. :ninja:
Anyway, to give an actual explanation for why: the reason I'm thinking it might have been that particular Devil Fruit is because of the whole "voice of all things" Rayleigh brought up back at Sabaody. Now I've seen that a lot would attribute that to the Observation Aspect of Ambition, which without looking at anything else beyond Rayleigh's statement would be a good guess. The problem I see with that is it doesn't take into account the other thing he mentioned about Roger being incapable of reading the ancient language. Attempting to link that line with the Observation Aspect as well is a bit more shaky, but partially workable if you can explain things properly.
Where that falls apart is that we know that the Roger Pirates left a message written in that same language beside Shandora's poneglyph. I don't think it would be an unreasonable assumption for Roger himself to have been the one to do it. Hearing "the voice of all things" is all well and good, but actually being able to leave a whole written response to anyone that may find that poneglyph just seems ridiculous without some kind of Devil Fruit since they're the most out there thing in the comic's setting. Include the whole "become enlightened" result of a human eating the Hito Fruit, and maybe gaining that state is what allowed Roger to decipher what was written down in poneglyphs and even the knowledge to write the ancient language.
Basically Roger just "knew" what the information contained was, to him it may have been as simple as breathing. Which explains Rayleigh pointing out that Roger couldn't even read the runes to Robin.