That term drives me nuts too. Partner is someone you’re in business with, not in love.
Or you’re a cowboy.
Howdy, pardner!
Influential publishers develop standard style guides that its writers must follow, which is how some of these wording innovations get popularized. The link lists a couple dozen major writing style guides.
Back in the 70s and 80s when living together and sex relationships without marriage became more common, US publications were attacked for "infantilizing" women by captioning them as "girls" or "girlfriends" when women were trying to find roles not limited to homemaking. UK newspapers chose “partner” as a term that would be neutral for unmarried couples also including its numerous gay celebrities, clergy and members of the upper classes.
Musk's background in South Africa and Canada leaned British, so he tends to use British expressions. The OP originated on his site, X. The internet and the influence of online British publications like Daily Mail and The Guardian have spread this term to the US.