For small indies it makes sense to publish on the pc epic game store a year earlier to get fundet for simply releasing the game there earlier.
A lot of smaller indies used that time as a kind of early access and got later super popular on steam, the main pc game store.

But for a big studio to do something like this is scummy
They already have a playerbase and a big budget
I dont even know if payment was involved but there is no other reason i can think of that explains this