From reading this I think that maybe people have forgotten that Multistrike and Cleave are 2 separate trinkets
either that or those words are being used to describe each trinket!
AoE scenario with corruption on all targets:
Multistrike trinket: each corruption has a chance to damage the target it is on, again (1/3?)
Cleave trinket: each corruption has a chance to damage targets around the target that that corruption is on.
Think of Multistrike like elemental's mastery, and Cleave like a mini-Nether Tempest.
Thinking this way, Cleave would be better for AoE (in fact it would get better the larger the pack) because as the number of corruptions increase, so does the chance of one of them cleaving. Whereas Multistrike's damage doesn't improve with more mobs as its effect only hits the target.
However, if Cleave's proc rate is far less than Multistrike, then that Multistrike damage might be more than the occasional cleave hit?