They've started a big push marketing the game, they obviously want people to stay excited for what it is, rather than thinking about any downsides. His answer doesn't mean they haven't thought about it, it means one of two things: either they are yet to come to a decision, or it won't actually be free-to-play. Either way, answering that in this interview would have been a moronic step to take, so he changed the subject away from it, and onto "we care more about games than money".
I love Valve, they're a wonderful company, but they definitely think about monetisation right from the start of a project - they're not idiots. And making money from a game doesn't have to mean the game isn't good.