As far as I can tell (this isn't gospel, just something I've inferred as the only reasonable explanation from comparing the codebases), Apple reconstructed a Mach 2.5 style kernel, like NeXTSTEP had, from fresh codebases (OSF Mach and 4.4BSD-Lite2), perhaps because 4.3BSD was still encumbered at the time.

The Darwin 0.1 and 0.3 releases contain the old kernel, derived directly from NeXTSTEP, and that's the direct derivative of Mach 2.5. The later XNU appears to be a reconstruction of that kernel with unencumbered code and that's also when IOKit replaced DriverKit.