Keys are sold out for now - have to wait until it comes out on Steam...
If that were the case then nothing could ever be sold out, digital or otherwise, because you could just make more or wait for more to be delivered from the manufacturer:
Store: "Oh, we're sold out of apple juice right now."
Customer: "Don't tell me that, you can order more or the orchard owner can pick some more apples, press them and make more juice, and then ship it here. So you can't be sold out..."
Sold out simply means no longer available.