They do some nice tricks to graft the XNU code into Linux, but it's
conceptually quite similar to the foreign ABI layers most *NIX
systems
have, although they have per-thread kernel-ABI contexts to allow a
single program to make both XNU and Linux system calls, depending on
where it is. The other quite neat thing is their library that allows
calling between libraries using slightly different ABIs. This is not
a new technique either (Transitive / Manchester pioneered a lot of
this work over a decade ago), but it means that you can use native
Android libGL (for example) from an iOS app.
It's a research project not a serious product though, because it
relies on using a load of Apple libraries that they don't have a
license to use on Android.