Assuming this was the case (personally I have no idea if it was) then it would stand to reason that Joss was operating from his Avengers template as this would be the reason that the suits parachuted him into it in the first place. My own theory is that Snyder would have presented his 4 hour raw cut and then negotiated it back to around 3 hours which was the established length of Avengers Endgame, so there is a precedent there. A lot of the slow-mo and drawn out scenes we saw in the Snyder Cut where he had creative freedom and were enhancing but not strictly necessary to maintain the atmosphere could have been easily shortened to claw back minutes of time without compromising the overall story he wanted to tell. I wouldn't be surprised if this is the way the game is played by directors, to build in a lot of atmospheric bulk that can easily shed for the theatrical cut but saved for a later extended cut for purchase or streaming. This way they don't have to compromise their overall vision, particularly when they are a director with a lot of leverage and veto.

tl;dr Basically, my guess is that when a family tragedy forces a top-tier director to exercise their instant exit clause so late in the development process the powers that be suddenly found they had full creative control back in their hands. So they used the opportunity to drop the director of their choice into the seat with strict contractual guidelines as to what the final product had to be.

My guess is that the studio wanted to take the safest path-to-profit. And that was to focus the film primarily on their three marquee characters: Batman, Wonder Woman and Superman who are already recognisable and marketable products. Aquaman had an up-and-coming movie in post-production at the time so shoehorn him into the movie where possible to introduce the character, oh and give him a comedic side during the reshoots while you are at it. The Flash - he's not big and important enough to give him the most pivotal climactic scene in the entire movie so even though we've spent millions already filming and finishing those scenes let's toss them out to save time and let the big three have the lion's share of glory here. And Cyborg? Just use him as little as you can to drive the plot, you can save plenty of time in cuts right there. Oh btw don't forget to toss out that dumb "Martin Manhunter" crap at the end, never heard of him and nobody cares anyhow.

So yeah in my mind that's exactly how it might have played out, right down to them not even knowing a characters proper name.