What did you honestly expect? Once news of a game giveaway ends up in feeds, forum posts, and places like Reddit communities there is going to be a flood of people coming, especially for a game that looks to have some actual real production quality to it. I can certainly understand your concerns but you are incorrect about Humble not giving free Steam keys... they do all the time.
Anyway, like someone mentioned in another thread a way to do a giveaway without distributing keys is to use the functionality that exists in the Steam API (Oauth??) to redeem an activation key directly to a user's Steam account with their approval. Using this method the game can only go directly to their account and they will never see any activation key.