To be clear, I sign up for humble account then pay $20 for the entire bundle. Seems like it's too good to be true. No catch at all?
I'm assuming you've never used Humble Bundle before?
You essentially give an amount equal to or greater than the amount required to get the tier you want. So for this bundle there's a $1 or more tier, a Beat-The-Average tier (which tells you how much you have to pay to unlock it) and a $20+ tier. And you get every tier under the one you paid for. So if you buy the BTA tier you get the $1 tier and the BTA tier. If you buy the $20+ tier you get all three tiers available.
Maybe I'm just not processing this because the deal is so sweet, but $20 for everything??