The steam key part is what is publicly available.

However, Wolfire, the developer of Overgrowth tried to sell non Steam key versions on other store fronts at a lower price due to those stores having a lower. Valve told them Overgrowth would be removed from Steam if he proceeded in doing so.

They are involved in an ongoing class action suit against valve over this. If this were just a misreading of the terms, it would not have been an ongoing suit for the last couple of years.