Yeah, it's not unlikely they'd buy the development team (Vigil) as well as the license, as was the case with some of the other sales. Remember, THQ is just a publisher, and what they're selling off is the development houses they've funded, in addition to the IPs. I think the main reason none of the other companies were willing to buy Darksiders was that Darksider II have little earning potential left, and a third one was probably not even in development yet. As such, all they'd get for their money was the privilege of funding Vigil making it.
I think you're also boxing platinum games in a bit too much. A lot of developers are fully capable of making a diverse range of games. A lot sticks with the same genres so they can build on past experiences, but getting that experience can be as simple as hiring a new lead developer.