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.
Oh yeah the film wasn’t meant to have a non Imax release in 4:3. Snyder has said he wanted the film in 1.85 for all other releases but since he wanted to give his fans the full Snyder Cut experience he went ahead and included the 4:3 for the HBO Max release. The 4:3 is less of a “rush job” and more just him trying to please his fans. Whether it was worth it is up to debate. For me I think he should’ve made it an Imax only thing and made the other releases 1.66 or 1.85.