That movie began principal photography in April 2016, only weeks after Batman v Superman: Dawn of Justice was release and met with a lot of critical derision.
Take Justice League, which was released in November 2017.
Even if Zack Snyder still ended up departing following the main shooting period and Joss Whedon still oversaw reshoots, pushing back Justice League to sometime in 2018 would have at least allowed any needed changes to the script to be made more slowly and carefully, rather than rushed through.
There are plenty of instances in film history where a blockbuster would have been better served had it been delayed rather than forced to come on its original release date.