At least, in terms of this article, a start-up that has growth ambitions and has just raised a round is a great bet. Unlike a potential investor, you didn't need to get in before that round to make the investment worthwhile, and in a sense you don't care as much about the company itself being an outsized success: not if your primary goal is around learning, which I think is legitimate, especially early in your career.
Similarly, I suspect it's not quite right to assume lessons in one start-up don't apply to others. In London at least, there's a strong network of people across the start-up scene who talk a lot. Many of those start-ups have very similar cultures and ways of working, similar enough that they hit the same problems and lessons learned it one often translate to others.
My intention, writing this article, was to encourage people to view joining a start-up with momentum as a valuable learning experience before they start their own thing, possibly more valuable than a big corporation or just jumping into start-ups themselves.
I think finding companies that can provide you with that growth isn't too hard, and I'd be happy to help anyone who wanted to find them, if they wanted it :)