The way I see it, after every other attack, it "pretends" to do another attack and then just applies the on hits that attack would apply. That way, sheen procs on the actual attack, you lose the buff and thus it doesn't activate on the sated devourer on hit application, because it's already gone.
Also it's not just that, but there's no sated devourer proc at all when using morde's Q. It takes 5 attacks for the first purple ghost thingy to appear.
I'm going to try a game out as devourer xin right now to see if it actually doesn't apply the Q damage when using devourer because I doubt it doesn't. And even if it doesn't, there's a difference. Morde's Q applies physical and magical damage, the physical doing lifesteal and on hit effects, the magical doing spell vamp & on spell (like rylai's)