UPDATE: Since the Screenrant article has been published, the WB has removed both Snyder and Whedon’s name from the logline for Justice League, so we are assuming that the final directing credits will most likely to be announced closer to the movie’s release in November. I'm going out on a limb and say someone who works for WB screwed up or did, for some odd reason, did it on purpose. To the best of my knowledge, it'd be literally impossible for Whedon to reshoot enough of the movie to claim a credit under DGA rules. I believe you have to shoot more than half the film.
Here's Screenrant:
"Back in May, director Zack Snyder stepped down from the film due to personal reasons, with Joss Whedon (who was then working on Batgirl) stepping in the oversee the already-planned reshoots and the rest of the in-process edit. The Avengers filmmaker has been rather forward about how he’s maintaining Snyder’s original vision, although it since emerged he had a hand in the re-scripting that led to the reshoots. Through all this there’s been the question of whether this involvement will be enough to warrant a directing credit. Now we have an answer."
"The Justice League Comic-Con Sneak Peek was posted to the official Warner Bros. UK YouTube account with an updated logline for the film that described it thusly: “A film by Zack Snyder and Joss Whedon starring Ben Affleck, Gal Gadot, Henry Cavill, Jason Momoa, Ezra Miller and Ray Fisher.” That Snyder and Whedon are sharing ownership would seem to confirm that the latter will indeed be credited on the film."