Oh, I agree that Darwin is, especially by way of NeXTSTEP but even independently since then, significantly derived from the BSDs (IIRC mostly FreeBSD, but also a chunk of NetBSD code got used to make it). I just think that that would have happened with or without the PC being around. In some parallel universe without IBM making the PC, the unix family still prospers, probably mostly on the 68000 family instead of x86, Apple does their own thing for a while, and then those lineages converge in Darwin just like in our universe.