I agree with the sentiment of the letter but why does Sal have to provide the optimal video lecture (code) for every topic (module) for his project (Khan Academy).
As a single person, even with a team of advisers, he will not be able to do this for every topic he covers.
One solution can be that he provides a clear interface to each of his modules and sub modules e.g. this module should cover changing base of logarithms and the student should be able to pass these questions (units tests) in the end.
In this model people can contribute in two ways - either make unit test more comprehensive or make the teaching material (code) better.
As a single person, even with a team of advisers, he will not be able to do this for every topic he covers.
One solution can be that he provides a clear interface to each of his modules and sub modules e.g. this module should cover changing base of logarithms and the student should be able to pass these questions (units tests) in the end.
In this model people can contribute in two ways - either make unit test more comprehensive or make the teaching material (code) better.