You can't get the Steam key you were promised, you have to log in after clicking the buy now button at the top, and then login to your Steam account through itch.io site.
This seems like fraud. That is not a Steam key, that is an activation after you give login details to a third party site. A Steam key is a 16-digit alphanumeric string, that when entered into Steam, redeems a game on Steam, and that key is no longer valid. You cannot look at any Steam 'key' on games you own, it is for activation.
All the language both prior and current says you will receive a Steam Key. After thinking about this more, this is absolutely fraud, and opens up both the dev and itch.io to problems with credit card processors if they should be alerted to this fraud.