That was back when there was "real" UNIX around, as well as a number of clones, including Microsofts own Xenix (maybe they had offloaded that to SCO by then). So UN*X was one way to indicate that it meant UNIX-like OSes.

Turns out SCO bought Xenix in 1987, but Microsoft was just a couple of years removed from being the biggest Unix vendor around at this point.