How can someone ever think that Affleck is replacing Keaton? This movie is set before The Flash, probably leading straight to that film. Keaton's return as Batman has been the biggest selling point of the DCEU for a long time now and he's not only having a huge role there but a probably even bigger role in Batgirl which is also deep in post-production.
Announcing Affleck's Appearance in Aquaman, in what will likely be a very small cameo, is probably just so that some of the negativity over Heard stops and people start focusing on that instead.