You have to remember that allot of these sales are from key retailers that will do stuff like buy keys wholesale. I’m not completely sure how that market works honestly, but that generates a lot of sales.
The case is really going to depend on David proving that Valve engaged in the alleged behavior. Games go on sale all the time, so there is a difference between a game being temporarily cheaper on one site because of a sale or key resell market demand, and valve using their influence to fix prices.