I don't want to derail to a swarm discussion, but arrows can damage swarms just like any other weapon damage. Are you considering the bane damage an "effect" (another ambiguous term) as referenced in the rule below?
"A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate), with the exception of mind-affecting effects..."
I'm guessing you would not allow fire damage from a flaming sword (or arrow) to damage a swarm either? (It looks like that is another controversy)
A Dimunitive/Fine swarm is immune to the weapon damage of an arrow, therefore the bane extra damage as well. That's what he's referring to.
I don't want to derail to a swarm discussion, but arrows can damage swarms just like any other weapon damage.
Yeah they meant fine/diminuative swarms.
A swarm made up of Tiny creatures takes half damage from slashing and piercing weapons. A swarm composed of Fine or Diminutive creatures is immune to all weapon damage.
And no, shooting a Swarm-Bane arrow aat a Diminuative Swarm wouldn't harm it, neither would it harm a creature that is immune to Piercing damage.
It's worth noting that you can buy arrows that deal bludgeoning damage so if the character is prepared they can overcome this obstacle.
It's also worth noting that it should be VERY obvious if an attack deals no damage. There might be some corner cases, but someone who unloads a stack of bane arrows to no effect shouldn't be oblivious to the fact.