For Burn Redeems with unlimited supply, the Airdrop feature simply counts as an additional mint.
For Burn Redeems with limited supply, two scenarios are possible:
If the supply after airdrops is not yet reached, the airdropped tokens add to the minted count.
If the supply after airdrops exceeds the previously set max supply, then the max supply will increase to accommodate airdrops, and be reached. (ex: if I set my max supply to 10, 5 of my collectors redeem the tokens, and then I airdrop 10 additional tokens, the supply will increase to 15 and the redemption will close with 15/15 minted).