I'd say their mention also raises the question of how trolls were able to secure the top third spot for most dangerous predator over them. Or even dementors, for that matter. A threat you don't know to run away from until it's too late is worse than one that you can't defeat, but can easily escape.

Possibly a single imp will eat its fill from a human being without ever doing fatal harm. But a few Imperiused imps would be an absurdly deadly weapon.