Yes, that person said "most devices use Ethernet", to which you correctly pointed out they meant to say RJ45. However, in the process of making that correction you made an unrelated error yourself in saying that fiber is Ethernet too.

>you can by no means assume copper when talking about an Ethernet port

You can, if by "Ethernet" you mean an RJ45 jack and its cable, which is a fairly common usage of the word. It doesn't matter that it's technically incorrect. The default idea of physical protocol that the word "Ethernet" invokes in most people is that of twisted pair copper cabling. If you take a random person and put in front of them a fiber optic cable carrying Ethernet and a copper cable carrying serial signals and tell them "would you mind unplugging the Ethernet cable?" they'll disconnect the copper cable.

>Your assertion that "everyone will understand" is also something I plainly know to be untrue in my bubble.

Oh, so you know several people who when presented with the dichotomy of Ethernet or fiber (because that's what the comment you replied to was about, as it put Ethernet in contrast with fiber), they'll be completely dumbfounded about what is meant, as if hearing gobbledygook?