https://en.wiktionary.org/wiki/gold_in_them_thar_hills

"There's gold in them thar hills" is an old phrase, probably a misquote, but has been long been associated with gold prospecting in the US old west.

So a non-binary gendered gold prosector found lots of gold in "them/their" hills is a reference to how people of non-binary gender tend to use "they/them" as pronouns.

"Them/their" isn't exact, but their is the possessive form of the third-person plural pronoun "they." So it's still referencing to correct pronouns for non-binary gender, just shifted slightly for the sake of a joke.

"them/their hills" sounds like "them there hills", which is a different (somewhat improper) way to say "those hills".

It’s a play on how prospectors / old timey / uneducated people would talk. Imagine a hillbilly accent with poor grammar.

It's a play on an idiom, "there's gold in them there hills." I'm not sure if it's just a really old idiom, or just a rural dialect, or both.

https://idioms.thefreedictionary.com/there%27s+gold+in+them+thar+hills

While the layman would say, "They dug up a fortune in those hills.", a gold prospector, likely familiar with hick speak, would say "They dug up a fortune in them there hills." The non-binary part comes into play by turning there into their, alluding to the assertion of pronouns common with the non-binary community.

I second this.