This is a preposterous claim that requires a lot of revision to be even close to true. Marketplace owners have been using their own games' exclusivity to push their platforms since Steam's introduction. Even just talking about third-party exclusives (which IMO is an arbitrary distinction — they're just as exclusive and for the same reasons), Valve paid for third-party exclusives early in Steam's history. Once they got rich enough, they started just buying companies outright when they wanted exclusivity on a game somebody was making, which is a much stronger exclusivity deal than the relatively tepid "the game comes out a few months earlier on our store" deals Epic is best known for.