I believe that focusing on a single product is an unbelievable learning experience honestly, and is the real gain for designers, but your point is still valid.
There are many ways to gain understanding of how to design for startups and if anything. By working on so many startups, some briefly, some all the way to series C funding, some to exit, I have learned that there are no one way that is the right way.
Also mind you I still have had to build my own company (from 2 people to 60 people back to 30 people and from 2 partners up to 8 partners and back to 4 partners) so you can say that I have stayed focused on that.
