They think it's an emerging technology and want to be at the forefront? Hence building the Vive with HTC and working on their tracking technology. Gabe Newell has been waxing lyrical about their VR plans just this week. They are currently making three VR games.
Reading the original source article [1], I think he's enthusiastic enough. He also comes across as being more conservative than many at the pace that it's going to be picked up and realistic about how limited the tech is currently.
Per your quote, he apparently also is open to the possibility that VR could turn out to be a dead end but that's not so much a lack of enthusiasm as not being a mindless cheerleader.
I think your assessment is spot on. I think his conservative take probably stems from the fact that he and Valve are going to be okay no matter what happens with VR. Almost nothing is at stake for him.
