/lit/ - Literature

So I've got this essay to write that's due on Monday. No big issue, I'm not here for help. However, I found something I think you all would find interesting. My professor is a very strict person who actually issued us all a copy of what exactly he wants addressed in this essay, and style sheet we must conform to. He will brook no thesis, he told us. Just answer the question. Anyway, one of the *requirements* on the style sheet is the following:

'Avoid sexist language by using the plural form ("Historians sought") or by referring to "people," "citizens," "residents," or "Americans." Nations are gender-neutral - "it," not "she."'

Now, am I mistaken or is this constrictive and violates a law of grammar? I was under the impression that in a situation in which a person's gender is not known, it is grammatically correct to refer to that person as "he"