Ashe's Q doesn't actually proc on-attack effects 5 times.
Ashe's Q says each flurry procs on-attack effects 5 times and on-hit effects once. On-attack effects are those like Guinsoo's stacking passive, Shiv's stacking passive, Runaan's passive, Sated Devourer's phantom hit.
Ashe, in theory, should be able to stack Guinsoo's with 5 attacks (4 + one Q flurry). That doesn't happen. Shiv should gain stacks five times per Q attack, it doesn't happen. Sated devourer should proc every Q attack, doesn't happen. The only interaction that does work is the one with Runaan's.
I'm not saying she should be able to do all of these, but saying that her Q procs on-attack effects is just false.
PS: seems like the Wikia page for Ashe was edited and the part saying she procced multiple on-attack effects got deleted.