There is generally a good understanding of what pronouns are genuinely held by people, in much the same way that freedom of religion doesn't mean that you can simply make up a religion and expect to be entitled to all the same rights and protections as established religions. Not everything that everyone SAYS is a personal pronoun would necessarily acquire the protection of the law in the same way. It would really not be any different to any other laws which touch on modes of address, such as using an abusive nickname, or even refusing to use someone's preferred name with the knowledge that not doing so was harmful to them. Both personal history and evidence would be taken into account should a breach of the law be alleged. Trying to apply broad, sweeping standards to laws like this is nearly impossible, but fortunately things like this are not really legally dealt with in a sweeping fashion, but on an individual, case-by-case basis.