The ship has sailed on this one. Arguing that UFO means "unidentified" (i.e. probably not aliens) is like arguing whether the Democratic People's Republic of Korea is democratic or a republic, or arguing that the Nazis were socialists because it's literally in the name (national socialists). Not only can words change meaning over time, labels can and sometimes do actually contradict their literal meaning.

I remember when there were huge online discussions about the incorrect use of the term "HTML5" once non-technical people got wind of it, and before that over the use of "internet" versus "web" (and whether it's "the internet" or "the Internet") and of course everyone knows the copypasta about "GNU/Linux" and maybe even the discussions about what a "meme" actually is, etc etc. None of these discussions are fruitful and by insisting on an older dictionary definition over the de facto majority use of a given word you're not demonstrating superior knowledge, intelligence or expertise but quite the opposite when it comes to language development and human interactions.

Words don't mean anything except what the two people using them to communicate with each other assign to them. In one-to-many and many-to-many conversations this means the majority use of the term generally prevails unless the minority use provides immediate superior utility (which is how a lot of slang enters mainstream use). It's all just sloppy lossy soundwaves and unicode strings thrown in each other's general direction - that we can transport ideas at all is nothing short of a miracle of nature.

this jordan peterson style semantics game is getting old, i think we all know perfectly well what "believing in UFOs" means, and asking the questiong you are asking muddles the discourse