it is possible to change your philosophy after some time.
If Zack really did change his philosophy, he did it after he filmed 90+% of the Justice League, because the majority of his footage was clearly shot with the intent to letterbox it, probably to a theatrical 2.39 (same as BvS). The brand new VFX shots were, however, made with the intent for open matte. So his change in philosophy probably had more to do with getting this release out the door quickly than suddenly deciding that 4:3 open matte is the wave of the future. The fact that they put Batman’s voice SFX onto Aquaman by accident shows this was kind of a rush job.