A reasonable interpretation. The FAQ says not that immune enemies can't be targeted, only that they can't be targeted by player card effects -- you *can't* attack an enemy, immune or not, without targeting it, but an immune enemy *can* be targeted for an attack if they are engaged, even if staging.
So the question then becomes what the "when doing so" means on Dunhere's card. If it means "When Dunhere targets enemies in the staging are and attacks alone", I think it should qualify. If it means "When Dunhere *uses his ability* to target enemies in the staging area and attacks alone", it definitely doesn't. And rethinking this, I agree the latter is more likely to be the intent of the card.