I mean, my only real argument is about efficiency and language exactness for the sake of clarity of meaning. Here’s a version based on my suggestion:
“This person understands pronouns, if everyone was like her then no one would use they are them to refer to one person.”
I’m not attacking pronoun users, I’m advocating for more efficient pronoun usage rather than arbitrarily requiring others to redefine their pronoun usage for every single individual that wants to use a different unique pronoun. Why, you ask? A pronoun is a shortened identifier than can be used in many different instances to represent a noun, in general, individual pronouns are a substitute for individual names.
Your name is the actually unique identifier more-so than any pronoun is or needs to be.