Is it not a colloquial use which was frowned upon by grammarians until gender-neutral language and political correctness became fashionable? In my language, French, we would always use "he" about a person whose sex is unknown, the masculine functionning as a neuter (grammatical) gender in that case, and it's the same in a few other languages I know a little.
Maybe we should all speak Persian, for instance, where there are no genders, and fight for the eradication of Telugu, where there are two genders:
— the higher gender for one or several men and for several women but not one;
— the lower gender for one woman and for one or several animals or things.