mmap isn't "serial", the code that was using the mapping was "serial". The kernel will happily fill different portions of the mapping in parallel if you have multiple threads fault on different pages.
(That doesn't undermine that io_uring and disk access can be fast, but it's comparing a lazy implementation using approach A with a quite optimized one using approach B, which does not make sense.)
(That doesn't undermine that io_uring and disk access can be fast, but it's comparing a lazy implementation using approach A with a quite optimized one using approach B, which does not make sense.)