But why'd he make two different versions of the same movie?
A shorter PG-13 version of the film is friendlier to the existing Netflix algorithms in place.
One of the reasons for shorter season that we've been seeing lately, besides a budgetary consideration, is it appeals to the metrics that tracks a series/film completion rate. Streaming allows you to take hard data on how fast viewers are consuming something, and if they are finishing everything presented to them. Lots of shows can get a decent sized audience for it's pilot episode, but a heavy fall off could now lead to a fast cancellation. Rings Of Power getting huge numbers for it's first two episodes means little to nothing. If people are staying up late and binge watching everything and finishing every episode, that's a positive sign for viewer retention for sequels, spin offs, more seasons, merchandising, etc, etc. Starting off with a 3 and a half hour version of Rebel Moon might have just broken a lot of casual viewers.
Snyder is attempting to rebuild a new "universe" Netflix is betting money on Snyder creating a new franchise that includes animation, TV shows, collectibles, graphic novels, etc, etc. That's the hope of any new story with lots of linked in world building. The standard bearer of course is Lucas and Star Wars. It's a cash printing machine. A longer version of Rebel Moon will naturally imply more material to stock the "universe" Imagine if the original Star Wars trilogy had an extra 15 minutes per film. We got 15 more minutes in a different Cantina, or another subsection of Cloud City, or some shipyards outside of Endor, etc, etc. Imagine how many TV shows, collectibles, video games, spin offs, graphic novels, etc, etc that you could pump out from all that material.
He's just making this harder than it needs to be. The theatrical release of Aliens is an incredible film. The extended cut/director's cut took everything great about Aliens and added depth, character development and context that only made everything so much better. Snyder just needs to make a fundamentally sound solid entertaining 90 minute movie to start. Then make it bullet proof as a concept. Refine it. Then and only then add in things like fan service, scenes to drive merchandising, more action scenes, UST ( unresolved sexual tension), spectacle, etc, etc. Along with more character development, B storylines, longer beats in existing scenes, etc, etc.
Instead he took a concept that would work best as an immersive video game, and tried to shovel a 1/8th of it into a single film that doesn't even functionally operate as a pilot episode. There's a good story here, but you aren't entitled to asking your audience to watch something twice.
Basic rule of film making - You are infinitely better off having a simple plot but entrenched with interesting complex characters rather than having an overly complex too broad plot with increasingly simplistic non relatable characters.
Can you get away with the latter? Sure, if it's packaged around pure fan service. I have a soft spot in my heart for Sons Of Anarchy, but the last 80 percent of that show's run made close to non sense at all. But it was fun and it had loads of fan service. But it definitely buckled in a bad way when Kurt Sutter tried to turn it into a dollar store version of Shakespeare. If Snyder insists on giving video game level characters in Rebel Moon ( I wouldn't assess he's done this in all his films, I quite love Man Of Steel myself...) , then just make a video game first.
I don't know if an extra 60-80 minutes of footage can fix the clear structural problems in Rebel Moon. Is it possible? Maybe. I wouldn't say this is a plan to fail, but I sure wouldn't assess this project as a plan to succeed.
