So... The functionality exists and is currently in testing, it's only just become stable enough that we're hearing about it, and they're notifying developers targeting the current service as far ahead of time as possible? I mean, it won't do anything for stuff that's too crappy to take software updates, but that's an entirely different matter.
Edit: Finally went and read the announcement. They're really not giving people much time, are they? Nevermind, three months is pretty bad.
No, this isn't a case of moving functionality from Nest to Google. They've removed the Nest API ("set the temperature", "set to away", "what's the temperature?"). They say "move to Google Assistant" but Google Assistant doesn't let you do that.
Also - I could sort of understand it if they provided a proper IFTTT style system in Google Home but they don't. All you can do is trigger actions based on a custom voice command or a time.
Edit: Finally went and read the announcement. They're really not giving people much time, are they? Nevermind, three months is pretty bad.