The reason Zack never changed the footage and his ideas for Mera’s backstory in accordance with Aquaman in the Snyder Cut was that he intended his Justice League to be separate from the ongoing DCEU canon.
However, there is one important detail about Mera’s backstory as revealed in Zack Snyder’s Justice League, which is impossible to fit in the shared continuity of DCEU post the 2017 Justice League.
But now, after the positive response towards the Snyder Cut, Zack Snyder is willing to complete his envisioned DCEU.
The fact that Zack Snyder’s Justice League deems Nereus dead, despite him appearing in 2018’s Aquaman contradicts the shared continuity of the DC Extended Universe.