He's criticizing them for selling out to Yahoo.
The point is not that "selling out" is bad, it's that you need to sell to someone who is committed to continuing the project in a meaningful way, and who has something to bring to the table besides money.
Speaking as a founder who has cashed out, I can tell you that the choice of buyer matters immensely, and that there are more important things in life than the amount of money in your pocket.
In answer to your last question: no, it's not that that one project was really more important than anything else I could ever do; but it took a few years to get past the bitterness to be able to focus, and find the right "next step"-- which could have been avoided, if I had sold to the right people at the right time (instead of the wrong people at the wrong time).
So: "selling out" can be a great thing, if done well. Or, a really soul-searing thing, if done poorly. Choose wisely. Here endeth the lesson.