That doesn't make much sense, XNU and the layers above it are very portable, they went PowerPC -> x86 -> x86_64 -> ARM64 after all. They also supported multiple different GPUs in the Intel era.
If the entire OS stack was open sourced today, we would have forks running on standard Intel/AMD CPUs in a week. They wouldn't have the same optimized power management, etc. But I think it would have a good chance of wiping out desktop Linux within a brief period.
macOS/iOS are part of the moat.