“I could just as well define “horse” as person who conforms to gender roles and then say that every Jew should be a horse.”
Except that there are legitimate definitions and usages of a term and some that are not legitimate. For example, if someone arbitrarily decides to take a word and give it a new definition that he just thought of on the spot and that no one else knew about until now, that is not a legitimate definition.
In order to be a legitimate definition, it has to either be a definition given in the dictionary or a common usage of the term.
Of course, if you decide to be like the main character in “Frindle” and spread your new definition of “horse” and it really spreads enough to be a common usage of the term, then it will become a legitimate definition.
But until then, it is not.