Page 17 of the Core Rules - Blocks Line of Sight (LoS)
"Abilities and spells cannot target any object or zone that is on the other side of a wall that blocks LoS."
I can appreciate the ambiguity. When designing this card I felt that "target" was a clearly defined term, but I can see where the wording of the previous ruling makes it sound as though "targeting" doesn't apply here. In order for the Wildfire Imp to teleport to a zone, he must target the zone, so if he does not have LoS to it then he is unable to teleport to that zone.
This is similar to the Teleport spell. Nothing about teleporting requires LoS, but the spell itself does because of it's target line. This is the same idea, his ability requires targeting.