(Darling requires a kernel module, which also isn't a thing you are able to do in the context of Docker as you are often just working with the host kernel.)
Wow! This is highly surprising and is totally unusual! They're embedding parts of the XNU kernel in Linux. I can imagine this was done as a last resort as there is pretty much no chance this will ever get upstreamed, not just because of licensing, but also simply just strangeness.