Llandru: It appears to be the Windows version of both SMAC and SMACX (you pick which to run when it starts up), running in a Wineskin wrapper. It is fairly elegant, and appears to look as nice as what Windows users get (being the same version) though nowhere as pretty as Boxer or Boxer-based GOG Mac games.
Thanks Llandru, that's helpful! I guess that means GoG are looking at porting Windows games, as well as DOS games; which means a lot of exciting potential!
I'd not realised this originally had a Linux release, Mephe. A 1999 Mac OS release isn't that relevant to getting something on Mac today as the underlying technology has changed so much. A Linux release could be though, which might be why they chose this as a first candidate for porting. Does anyone know if the original Linux release was native, or if that used Wine as well?