I always wonder how Docker works on macOS with a more UNIX-style kernel than Linux
It doesn't. Macos also uses a virtual machine for docker.
but is it really that hard to do Docker/OCI out of Linux?
Yes. The runtimes containers use are dependent on cgroups, seccomp, namespaces, and a few other linux kernel specific features.
You could implement a wine like project to run the linux binaries that containers contain, and then run some sandboxing to make it be a proper container, but no virtual machines or virtual machine container runtimes* are easier.
Linuxulator, a freebsd project does the above.
https://people.freebsd.org/~dch/posts/2024-12-04-freebsd-containers/
*these are much lighter than a normal vm, I'll need to check if this is what macos does. I know for a fact docker on windows uses a full Linux vm though.
Lol. Lmao