What else could they be working on other than a new DKC- a new IP? That idea seems less based on solid reasoning from people and more wishful hoping given the demand for a new Retro IP has been around for quite some time.
And, well. . .what they said in the interview above.
If they hadn't said that I wouldn't lean so hard into assuming they're doing something new, but the implication is that it's none of the three things they mentioned if they'd love to work on all of them again but already had a project.
That said, it doesn't have to be a NEW IP, just not one of those three. The idea of them working on an existing Nintendo franchise that happens to not be one of the ones they've already done is equally viable, including plenty of franchises that wouldn't be fantasies for people.
Although I'm really not expecting Startropics to be it if they do pick an existing IP because it seems like more of a fan wish than the idea of a new IP is. A new IP at least has Nintendo's massive new IP kick behind it in theory, if perhaps starting a little earlier than the main wave.
Saying it can't possibly be anything other than DK is like saying it can't possibly be anything other than Metroid prior to the announcement of DKCR. They've thrown a curveball before, precedent is not evidence in itself in this case.