I think the point being made is that the loss comes in after the exclusivity deal is over and the game is available on Steam. If someone decides to buy a game on EGS over Steam at that point, developers could be losing out on revenue. An EGS sale, assuming the advance hasn't been met, gives developers 0% as Epic has already paid them for it. But if there are alternative places to buy the game where the developer gets their 70%, then that could definitely be seen as a loss.
Imagine epic sitting on prepurchased copies, the one year period is over and people starting to buy the game on other platforms.
That puts a smile on my face.