The term probably was coined much later--just as term "Doo Wop" was coined long after its heyday in the 1950's. Judging from the examples presented on this thread, what is called yacht rock today was at the time called soft rock.
Just like nobody today can create a piece of church music that can compare to a Bach cantata, nobody today (or before 1975) can create a true Yacht Rock song.
There are a number of sounds that nobody today can fully replicate, one of them being the Motown sound of the sixties--for one thing, the analog technology used at the time no longer exists.
It’s the same with Prog rock, a lot of bands call themselves prog, but you could never recreate the classic Prog of the 70s, of bands like Yes and Genesis.