Which sounds pretty unlikely, considering that the makers of Genesis deemed their creation an inflexible piece of equipment ("can't cram another byte in it"), and it was not programmed to create a planet out of nothingness as a default.
What the mechanism could
quite plausibly have been programmed to do is convert the Regula asteroid. After all, it was right there and available, and the scientists had not yet secured their desired "empty class M" world for the final experiments.
We further know that Regula was close to the point of detonation, as our heroes and villains had only recently departed it at impulse speed. The star that this Class D planet circled would also be conveniently nearby. In contrast, if the mechanism created a new star, one would have to explain what happened to the old one...
Since our usually detail-minded hero Spock calls the planet simply "Regula", rather than something like "Regula I" (the apparent name of the laboratory, then?) or "Regula IV", it might be that the star system does not feature other planets or planetoids of note, leaving Regula our only plausible option for the Genesis planet.