Sure, but we're comparing the efficiency of one of your program's single threads, because otherwise you could take that same argument you just used and turn it

Sure, but we're comparing the efficiency of one of your program's single threads, because otherwise you could take that same argument you just used and turn it around and say fine, just run another thread then with another io_uring... and you're still ahead. You have to compare at the smallest unit of control plane. At the same time, multiple threads for a single program introduce context switches which are becoming horrendously expensive compared to the sheer number of IOPS that modern NVMe SSDs can do. Thread-per-core designs built around io_uring are the future of IO on Linux.