Earlier quoted context omitted.
If the article is correct, they bought the team.
400 millions for a 3 person startup that don't even have a proper web-site and is not in wikipedia seems crazy. But then again Google knows a little more about future value than me.
There is a difference between Snapchat being worth $3 billion and Nest being worth $3 billion. The former gets the valuation based on users, the latter on talent and intellectual property.
Ditto here: $400 million is not buying you users, it's buying you raw talent and IP. Users can go off to another service in a blink of an eye - IP can't (talent can, but you can often structure the deal so that it won't for some time).
This could still be a terrible deal (I'm sure there are some people at Google still a little sore over Motorola, where the IP was valued far more than it ended up being worth), but for very different reasons.