> io_uring is not an event system at all. io_uring is actually a generic asynchronous syscall facility.

I would call it a message passing system, not an asynchronous syscall facility. A syscall, even one that doesn't block indefinitely, transfers control to the kernel. io_uring, once setup, doesn't. Now that we have multiple cores, there is no reason to context switch if the kernel can handle your request on some other core on which it is perhaps already running.