Affleck said he wasn't dropping out of The Batman on national television and fans took him at his word. He dropped out as director a few weeks later.
Affleck said Batman was the greatest role and that he'd play an ape for Reeves. He dropped out completely a couple years later.
People may not like to hear this, but filmmakers lie. Reeves' comment strikes me as one that was made purely to placate fans for the time being - especially considering that Justice League had not come out at that point. Affleck confirming that he was dropping out completely and Reeves confirming that his film would have nothing to do with the DCEU would have been PR suicide for Justice League. Anything Affleck and Reeves said during 2017 needs to be taken with a boulder of salt.
If I had a hunch, I'd say this film won't have anything to do with the Snyderverse Batman, and all Reeves has to say is that plans changed after Justice League (even though it was the plan all along). It's the PR game. You say what you need to say in order to keep fans happy NOW, and then later on down the road when the sensationalism has died down, the truth comes out. One need not look any further than how they handled Affleck's exit. This has already happened a couple of times on this project alone and yet fans still keep clinging to these words. The truth is in plain sight.