This just came up in relation to the Han Solo situation, bit is relevant here also. There is a Director's Guild of America rule that states the following:
As far as what this means for the anti-Snyder crowd out there, another portion of the DGA's rules says the following:
IOW, Snyder is the person entitled, as per this rule, to have the "final word" on Justice League's structure, tone, and composition even though he has voluntarily stepped aside and handed post-production duties over to Joss, and so the decision to turn to Danny Elfman to supplement the film's existing score was very likely made with his input, even if it was merely cursory.
The existence of this rule also puts paid once and for all the notion that Joss could come in and dramatically restructure the film, something that, as noted previously, he wouldn't have done anyway even if it weren't not already prohibited by DGA rules.