100% likely.

I think they announced this because they wanted to announce Metroid: Samus Returns.

Think of all the backlash if Nintendo said "Hey here is Metroid: Samus Returns a remake of Metroid 2" then goes on in the Spotlight without announcing Metroid Prime 4. Guarantee people would be like "Where's my Metroid Prime 4, I want the Prime series", " why isnt Switch getting a Metroid", etc.

Like the backlash when Pokémon US/UM got announced with nothing about the Switch except Pokken, which is probably why they announced what is likely the next generation for the Switch in the Spotlight.

Basically, because we want to hype up the Switch

I expect Prime 4 to be a BotW situation (well similar in how long it takes and how much information they share). They have to announced it , as others have said, to keep the pitchforks in the barns but i bet we don't hear or see anything significant about this game until 2019. For those calling for a 2019 release I think you are being optimistic. I hope I'm wrong ....

Or they couldn't announce Mario Oddisey before because it wouldn't come to WiiU and they hadn't revealed Switch, but on MP4 since Switch just released and they want to keep sales high and show commitment to a stable influx of games they took a gamble and announced early on.