I think it’s one of those “the NUC is Dead, Long Live the NUC” type situations. There are so many tiny pc vendors now selling models that just didn’t exist when the NUC was originally released.
NUC was an Intel thing, it's used commonly for any USFF PC but NUC was specifically a line from Intel and, I believe, they sold the name off to Asus. They stopped making them, but other people are still continuing to make USFF PCs with Intel chips.