Heads-up: I talked with Humble Support. They posted the warnings in the DLC store pages as well about them being out of keys. So far they don't seem to know when they will arrive, but they should be.
I'd guess since the game publisher only sold this game on Steam, they are kinda new to how to handle distribution through keys. Plus, well, the historical lowest DLC prices probably earned some purchases. :D

Short story: Keys will arrive, but nobody knows when. I'll check every few days on Humble. In the meantime, just don't mark this giveaway. You have the gift link already, so eventually the key will appear there. ^.^