This is not an example of someone being fired for saying or believing that there are two genders. This was someone deliberately misgendering and ungendering a trans boy (not non-binary) in class after having been ordered to stop.
A pronoun operates in a purely social context.
I would recommend reading "THE CATEGORIES WERE MADE FOR MAN, NOT MAN FOR THE CATEGORIES": https://slatestarcodex.com/2014/11/21/the-categories-were-ma...