My immediate thought was this: https://en.wikipedia.org/wiki/User-mode_Linux

More seriously, it is a great use of indirection insofar as replacing the full Linux kernel is a gargantuan task. A userspace kernel per task or task group.. makes a lot of sense. I'd be interested to know how it captures syscalls - eBPF? It wasn't obvious from a skim of the readme.

This kind of give-me-this-interface work seems pretty perfect for LLMs, too.