It makes some sense.... If Bethesda came out with their own announcement for Skyrim Switch.... with the hardware only explained via a 3 minute "teaser", Bethesda may be pressured to talk about features and content.
That can potentially affect the Skyrim product on PS4 and XBO (if say.. there was different or unique content on a Switch version). Bethesda would also be in the uncomfortable position of talking about Switch features that Nintendo possibly only wants to reveal later.
Imagine if Bethesda talked about "touchscreen functionality" (when Nintendo has chosen not to talk about touchscreens right now)... or if they had to explain how "back-to-back Switch consoles" multiplayer would work.....
It's best to follow the lead of Nintendo in that scenario and not say more than the hardware provider is willing to say.
Of course in that case, you might think it would have been simpler just not to include 3rd party content in the trailer? But you have to admit that putting Skyrim there generated some buzz by way of a "response" to an old criticism of Nintendo and 3rd party relationships.
It's also not wise to suggest that Bethesda (and 2K) can confirm everything and then just tease the public by saying they're going to play along with Nintendo's game and zip their lips on details. That could be misconstrued by Sony and MS as a form of "Publicity Collusion" and remember in Bethesda's case it is releasing content on both Sony and MS platforms at an imminent period. So it has to maintain a position as a "neutral supplier of content".