I'm trying to test it now, but after about ten attempts, Inver hasn't pulled out a single Pokemon that knows any Bug, Grass, Fighting, ground, Rock, Ice or Electric moves. I've had a Sandshrew whose only attacking move was Aerial Ace and a Rhyhorn that only knew Take Down, though. (What is it with him and only using NFE mons this time?)
EDIT: It's the first option. A Cacnea used Needle Arm and I got the "mysterious air current weakened the attack" message. In other words, Delta Stream actually does reduce the damage of moves that are SE against flying types, not just a preset list of types.
EDIT: It's the first option. A Cacnea used Needle Arm and I got the "mysterious air current weakened the attack" message. In other words, Delta Stream actually does reduce the damage of moves that are SE against flying types, not just a preset list of types.