@Ric, I think you are right there.
It fits my notion about preemt priority, but the thing with one sub-priority is unexpected and gives rise to the question if and how this impacts tail-chaining.
Will simultaneous interrupts be treated as one event, failing to call dedicated ISRs?
If not, will the order be non-deterministic?
Not that the probability is that high, but going with Murphy’s law, one should be prepared 
Given my own experiments about interrupt latency (for another thread) of about 1800ns from trigger to serve, an application with lots of high frequency interrupts might need to know.