Just to clarify, Argalad's ability says that "if this effect reduces the enemy's threat to zero, deal one damage to it"

So if another card effect (such as Ithilien Tracker) already reduced the enemy's threat to zero, does Argalad's effect not apply? I feel like it would because the enemy's threat is still zero, but I would just like to check.

I think Argalad's ability won't work if the enemy's threat is 0 (native or as a result of another card effect).

Caleb wrote this about the Lore Aragorn card, which also uses the word "reduce" :

"If you play Vanish from Sight in the refresh phase, you can still use Aragorn’s ability, but only if your starting threat was less than 20 because it says “reduce.” If your starting threat was higher than 20, then you cannot “reduce” your threat by increasing it."

With the Vanish from Sight event card, you are treating your threat as if it were 20.

Caleb is saying that to "reduce" something you have to make the value go down (which sounds obvious, but hey, card game and technicalities).

In the case of a 0 threat enemy, Argalad is not able to "reduce" the enemy's threat to 0. So no damage.

I think Argalad's ability won't work if the enemy's threat is 0 (native or as a result of another card effect).

...

In the case of a 0 threat enemy, Argalad is not able to "reduce" the enemy's threat to 0. So no damage.

I agree. It is already zero in that instance, so Argalad isn't able to reduce it any. I could see it working if Argalad's ability read as "If that enemy's threat is zero, deal 1 damage to it" but currently the damage is tied to the actual successful resolution of his ability to lower the enemies threat to zero.

Well shucks. Alrighty, thanks.