Yes, jet is black... coal is black, obsidian is black. That's why those words all mean "black".
Certainly "most" people are NOT referring to a possible ceramic white iPhone as "jet white", as that is idiotic.
Further, that is only how one SOLE individual referred to it himself in the original rumor (singular... there have not been multiple leaks of a white iPhone, nor multiple sources using this verbiage).
They didn't even say anything remotely like- "and Apple has decided on the term jet white", or even "they're calling it internally jet white"; the leaker merely said that themselves.
Now there are a few possible explanations- perhaps English was this leaker's second language & he thought that, because the jet black iPhone is shiny, jet means shiny. Or perhaps, caught up over posting his 1st ever tip, he simply misspoke in his excitement. We will likely never know for sure (and honestly.... who cares?).
There is one thing we DO know though: jet white is not a color, and never has been. Indeed, if you Google the term, and go page after page, you will find not one single reference to one single object referred to as "jet white", save the dozens and dozens of references to this leaker's verbal or mental slip up in which they (quite accidentally, I'm sure) tried to use "jet" as a descriptor of a shade of white.
Oops!
Why anybody would repeat that, unless quoting him verbatim, for the purpose of giving him credit for leaking the rumor of a glossy ceramic white iPhone, I'm unclear on.
Hypothetical scenario:
If I found out that Apple figured out a way to miraculously manufacture a clear Apple Pencil, and in my excitement to share that info- accidentally referred to it as "Clear Plaid"; I would NOT expect all of you to act as if you've lost your minds & only refer to it in a nonsensical term such as "Clear Plaid" forever- even if I was the original leaker. You could go ahead and refer to it as "the rumored transparent Apple Pencil", or "the rumored clear Apple Pencil", so you wouldn't sound foolish.
Is any of this tracking??