This is very much a cultural fit test. If people who use poor grammar are going to drive the author nuts, then as the the person running the company I think he's perfectly justified in treating the candidate as a poor cultural fit.
Additionally, as someone who employs writers as well as programmers, he might reasonably want to hire a programmer who is also a writer for the same reason that you'd want to hire a programmer who also has ops experience: it makes them a more appealing hire for the kinds of problems his startup is trying to solve.
Additionally, as someone who employs writers as well as programmers, he might reasonably want to hire a programmer who is also a writer for the same reason that you'd want to hire a programmer who also has ops experience: it makes them a more appealing hire for the kinds of problems his startup is trying to solve.
In short, I don't see the problem.