I don't understand why they were ever able to rival Byzantium as the Ottomans in the first place. That's akin to France rivaling Brittany, or England rivaling one of the Irish minors. If I had to guess I would suggest historical flavor but if you're at all aware of the time period the dynamics of the Otto-Byz relationship are apparent without giving the Ottomans a bunch of free power projection for beating up on a nearly helpless neighbor. This is a logically consistent nerf that is simply conforming to game mechanics.
I suppose this is a nerf to the Byzantines as well, but it's a small price to pay coming from the pocket of a nation that should be all but impossible anyway.
EDIT:
It does look like this change is bugged as welcome as it is. The Ottomans start out rivaled to Byzantium then "Surpass" the Byzantines at the end of November giving them a small amount of power projection. I could see something like this being intentional to mitigate the nerfed PP they no longer receive after taking Constantinople. Maybe a Dev can clear it up.