Maybe Payton was serious when he said that. But just because someone says they won't, doesn't mean they won't.
If they make it public knowledge they want Mariota and want to trade up for him, then everyone knows what they're going to do, and it is very likely that the price goes up. So by saying they won't, or dismissing it, it at least keeps people guessing, which outside of the Bucs in this year's draft is what teams tend to do.
That said, while they probably won't trade up for Mariota, it wouldn't surprise me if they did.
It would have been very simple to say we'll have to see how the draft goes, which is the pretty common company line. There's been no reports of the Saints being interested in MM, just speculation. Payton blatantly said, "We will not be trading up for Marcus Mariota." It's not very often you get such a straight forward answer from him.
I'm not saying they won't take him if he falls, which he very well could. But trading up for a QB of the future isn't where this team is at right now.