I don’t think “hun/zij” is invalid and I’ll happily use it for someone if they want it, but what I mean is that it doesn’t feel as natural to use it for a single person as they/them. They/them in English has a history of being used for singular people as well. Saying “someone lost their bag” is a pre-existing language feature. Unfortunately “iemand is hun tas verloren” doesn’t sound as natural and I’ve never heard someone use it like that. It seems to be common to just use the masculine pronoun “z’n” in cases where the gender isn’t known.

Again, I don’t mean to invalidate anyone, I’d totally use these pronouns for a single person if they prefer that. It annoys me that our language doesn’t have a clear neutral pronoun. But in my experience “hun” is exclusively plural whereas “their” has always also been in use as a singular pronoun next to its use as a plural pronoun.