Yes, they stop being pure-solo quick, if successful, when they're already a "started-up". Very early stage funding is at an earlier phase.
I'm curious about how successful a start-up can be, before adding anyone. [problem: founders who want to grow will do so ASAP; founders who don't, won't publicise their niche and we won't know about it]
Does Minecraft count as a "start-up"? With a billion+ dollar exit, it should be a "unicorn". I think Notch was solo for a very long time.