That's just the throwaway opinion at the end. The reason why you might want to consider this source as valid is that everything he/she describes about working at Valve matches everything we know about their studio and office guide/manifesto. They make it clear that at no point is Gabe going to swoop down and ask you to work on HL3 or something. You have to find your way, and that's what makes a HL sequel so challenging - you would need somebody at the studio (besides Gabe, I guess) that is:
1. Charismatic enough to attract other developers there with the right skillsets
2. Motivated enough to push forward a HL3 project
3. Convincing enough to have everyone believe that this project will ship.
4. Skilled enough in all the right ways to work on the project personally beginning-end.
5. Brave enough to do all of this knowing that if it ends up just being you, you're at the bottom of the stack and possibly out of a job.
They're not like a traditional studio; you don't just get 4 concept artists assigned to make a pitch. Presumably whomever was building the game would need to get it far enough along with a very small number of people, but quickly and to a level of excitement that it draws in a large part of the studio to see it to the finish line. Sort of like showing half of "Portal" functional or something.
I think the source is totally right on it getting more difficult every time, too. Why spend your time/risk your position within the studio if you're not convinced it will finish?
If there was some aspect of traditional development there - Gabe asked for it, jobs were secure, or said person was lucky enough to start out with an intrinsic designer/programmer team - I think it would be a lot more likely to at least get somewhere. As it is I guess it depends a lot on what the hidden political structure is.