a) is github monoculture is bad?
I really disagree with author's view on the emphasize on public github project in job interview process is misplaced. Yes, it will not eval how good you are at your previous job, but the problem is it is very very hard to find how good you are at previous job. Doing open source project at spare time is a positive sign of interest in programming. If in the interview process you said I don't have a github account, but I put all my repo at abc.com, I am pretty sure it will work, too.
And I don't care the monoculture if its service is the best in this field. Why use the second best if the best is possble? I would like to be an earlier adoptor if there are better service, even if there are currently no user.
b) how to disrupt github?
That I have no idea. What github does not provide but is really needed? I do not know. Would love to hear ideas.