Batman v Superman came out 19 months before Justice League was released and 14 months before Snyder was reportedly fired from JL. If WB was panicked over the lackluster performance of BvS, then they wouldn't have waited until May the following year and after principle photography on JL was complete to fire Snyder.
From what was reported, WB was concerned over BvS but allowed Snyder to press on with JL. Then Snyder showed them a cut of JL in March or April of 2017 that was allegedly an incoherent mess, after which he was dismissed from the project under the guise that he was grieving the loss of his daughter. That's when Whedon was brought in to salvage the movie.