re yeahbut
You are right. Here is the reply I received when I asked the author of the page the same question:
You raise an interesting issue. I have two answers. First, the power
of figurative language is in its connotations, not its denotations, and
so the factual accuracy of a trope is less important than its effect.
In other words, "longhorn" really could stand in for cattle generally,
and not just that breed. You could also refer to your car as your
"wheels" and be understood, even if your car had no wheels. Second, it
is not uncommon for indirect references to migrate in what they point
to. For example, "trampoline" used to be a brand name, "Trampoline,"
but its very success caused it to become a generic term (synecdoche is
precisely this trope of referring to a genre by naming a species within
it). The same thing has happened with Xerox (now a generic noun and verb
for photocopying), and Kleenex. Someone can say they've bought some
Kleenex even though they bought some no-name brand, and we would not
accuse them of being false. Why? Because we accept synecdoche as a fair
substitution for the factual name.
