Veracity of the rumor aside, StarTropics in and of itself is believable to me.
I mean, it's exactly like Kid Icarus, in that it's a very old NES franchise with one sequel, and more or less has just about the same amount of public perception (whether it was released in Japan is insignificant now - especially for a Wii U game developed by an American studio).
Also, it follows nicely with Iwata's seeming ongoing desire to establish new IPs by revisiting very old ones (Kid Icarus, Pilotwings, StarFox 64 (3D), Luigi's Mansion, and to a lesser extent Link to the Past 2, Yoshi's Island, and of course 2005's New Super Mario Bros.).
And furthermore, it ticks the box of being a familiar genre, with similarities to Zelda, Earthbound, Metroid and I guess StarFox too with the aliens, weird talking animals and space travel...