IMO, with Nintendo, it could mean anything. It could also mean that "our contract with Nintendo forbids us to officially reveal the support until they announce the first Game Maker titles on a Nintendo Direct". I mean, Nintendo kept DOOM a secret until less than 2 months before release.
It would be an incredibly weird inside-baseball thing to keep support for a game engine secret. They announce games on their own terms, of course, but I'm not really sure what the benefit would be of keeping this a secret. I would think it would be best to let developers know their platform options as soon as possible. I don't doubt the Switch will have GameMaker support eventually, it's just a bit disappointing it's taken almost a year.