The difference between passives and uniques vs basics and specials is, in general, that the former pair are mainly triggered effects, while the latter pair are actions taken by the toon.
The wording is that whenever a character uses an ability outside of their turn. Passive abilities are never used they are only triggered. Now on counters and assists something triggered that character to use that ability so it was an action (a use) that was triggered. With a taunt, it is merely an effect that was triggered. You can tell that no ability was used because no animation of the character happens to show it as such.
Bottom line is that passives/uniques are never used, they are simply always on and either have persistent or triggered effects.