Yeah, the explanation that I've usually heard for this sort of thing is that it's intended to get back CPU time that's lost when too many system threads are blocking to keep something on every core even during I/O (or pay for it in terms of the context switching overhead if you compensate for this with an extremely large number of system threads). The theory is that you'll avoid idle CPU compared to the common "one thread per core" way of doing things due to some of them being idle during I/O, at the cost of using some extra CPU to handle more things in user space. Obviously how much this helps can vary between use cases, but the measure of how much it's helping (or if it's maybe not helping at all!) is throughput, not CPU utilization.

This makes no sense. Epoll is already non-blocking, you never waste time waiting for I/O as long as there is work to do. Io_uring only boosts CPU efficiency (batching of syscalls, for example), it does not reduce blocking.