I do think you are right, they start out on gameplay; getting that inner game loop to be fun is primary (I kind of though that's how most (non narrative) game studios worked though).
they don't make games to tell stories
This was mostly what i was suggesting was incorrect. I also don't think it's a major part of marketing. I'm suggesting they don't bother putting the work of story into something until that inner game loop is fun.
TF2 was the result of experimentation with team based death match gameplay
Didn't they already know about team fortress? This seems off based on team fortress having already existed, same with wolfenstine enemy territory.
Yeah, i kind of agree with everything you've said, and history as i remember it kind of backs up what you've said about tf2.
But I don't agree that they don't care about story and only do it for marketing. I think halflife's episodes are all about an attempt at continuing that story.
I think that the Cave and Glados bits of portal are a large part of what made those games (of course the gameplay loops are really tight there.
I think the only way to know would to be an insider. I also don't think it really matters, the games they make are good.