For what happened here, Nichiporchik explains that scalpers bought copies on Steam priced cheaper in certain regions. If you release a game on Valve’s store and don’t set the price in each region individually, universal pricing creates discrepancies where this can happen. These were then being sold on less regulated marketplaces.

Regional pricing error caused the game to be much cheaper than it should be in some region. Resellers bought a lot of keys at that incorrect price to sell on shitty key sale storefronts at a profit.

Those keys are invalid by the Steam ToS.