When it comes to the "anime" style. Most of the time, when you see a female, her eyes/pupils are bigger than the male counterpart.
It conveys innocence and purity or something. It's not exactly a female only attribute. Anime just does this weird thing where sometimes the facial structure is absurdly different between the sexes.
And I guess it just engrains into your mind that this is how it is universally.
So whenever someone says ">draw a girl call it a boy" they're probably still not aware of this fact or perhaps they're willfully ignorant of it idk.