It is indeed a very wonky-sounding concept. A portable console that comes in three pieces, one of which which isn't even included when you buy the console - that's not an easy sell! This whole thing has the air of internet fan-created rumor about it, but hell, you never really know - maybe Nintendo has clued in to the fact they're about to obsolete themselves if they don't do something drastic.I'd say that's the only part of this theory that has the air of legitimacy to it.
Making a three-piece portable console also sounds weird enough for Nintendo, the company that did create both Wii U AND the original Wii (which, as we may recall, was almost universally expected to be a complete bomb, sales-wise), AND the Virtual Boy as well if we want to dial in the Way-Back Machine quite a few years now...
60fps gaming could be had on modern portable devices; iOS and the last two generations of iThings have its Metal API, and I would expect recent Androids (and whichever few devices that run it) to support Vulcan, yes? You might need to scale your assets depending on the hardware though, it'd mean additional obstacles Nintendo themselves isn't very used to during game development, but which is - let's face it - pretty common practice amongst most other devs.