I just actually watched the original video in which Gabe Newell said this. In the context he says it (start watching at about 40 minutes) it actually makes sense. He says Mac OS X is a lot more open than the PS3 is. When Valve thinks about developing on a platform, they not only look at the market share the platform has, but the direction they're moving in. What he means is that Mac OS X as a platform is ready for the future, whereas consoles are so closed that developing becomes less interesting.
What he says about the PS3 is that Mac is a more interesting platform for them, but it would be great if Sony continued on making the PS3 more open (like a Mac), instead of... (he waits a few seconds here) say, a GameCube.
It's a shame he got misquoted so badly.