And one of the crewmembers murdered by the cardassians on Empak Nor, and one of the Maquis in Learning Curve, and the barber on the Enterprise. Lots of bolian extras have always been wandering around.
And you're right, the Bolians were never firmly established as one of the founding races but heavily implied. Like when Kai Winn said "Would you sacrifice Vulcan? Or Bolias? Or Earth?" Or other cases in DS9 when Bolias is referred to as being very close to Earth. Closer than Betazed, farther than Vulcan. If Bolias was not one of the founding planets of the Federation, it is very close to Earth and considered one of its core races.