It seems to me to be rather disingenuous to state that the entirety of startups is writing code or that the code writing itself is to be the source of all of the learning. The major education in a startup is the fact that you are forced to, through whatever methods necessary (be it copious research, learning from somebody who knows better, randomly stumbling upon an answer, what have you) learn more or less everything relevant to the startup, from business skills to people skills to infrastructure to, yes, code itself.

Maybe this is insufficient as an alternative education (I'd really love to see some numbers for that but, then again, wouldn't we all?), but it's silly to bring it down to "writing code".