All tech reasons aside.
You dont need to work with marketing to understand that this film is produced by Sony.
No, it's not. It's a 20th Century-Fox property (at least until the Disney deal closes).
Jim can shoot with whatever he wants. But he does have a good history with Sony, having had them help design the separate body_and_lens cameras used on the first Avatar, as well as a number of underwater capable setups he's used on various projects. So it's not particularly surprising that he'd lean towards their products now, particularly with them being open to revisiting the separate lens unit design.
And, not to mention the fact that in the end, practical photography represents only a small fraction of what this movie ultimately will be.