Yep, I once tested (and verified) that PSI doesn't account for the asynchronous I/O completion waits (io_getevents) just like Linux system load doesn't. So, both of them would miss the asynch I/O completion waits as a thread waiting for async I/O completion is in S state (not D). Once problem is so big that the block device I/O queues get full and even io_submit() calls hang - these threads would sleep in D state, so would contribute both to system load and PSI.

Edit: I understand that PSI was created as an easy (and cheap) to query metric to see if your Android mobile device has a problem (and some apps need to be killed) and not really for full blown troubleshooting drilldown of server workloads.