This would be Strike 2 against Fisher by my count. First, he claimed that Whedon altered the skin tones of the black actors, but this was proven to be false and a retraction was made. Next, he claimed that Johns would be given the boot and that clearly has not happened. One more demonstrable false claim and I think Fisher will have lost any credibility he has. He should tread lightly or else he could derail whatever good was accomplished by his actions and inflict real damage upon future accusations made within Hollywood.
That said, Warner Bros. is one Strike 1 for falsely claiming that Mamoa was starring in a Frosty The Snowman movie produced by Johns & Berg. And that's on top of Hollywood's long history of covering up bad behavior on the part of producers, directors, executives, and stars.
It's pretty obvious that the production of Justice League was a train wreck and Whedon, Johns & Berg could have handled things much better, but given the ridiculous edicts given to them by now disgraced ex-WB head Kevin Tsujihara, those guys were under enormous pressure. They were given the almost impossible task of reshaping a film already shooting into a completely different film with a firm release date, a director who'd suffered one of the worst tragedies imaginable, a star with alcoholism, a second star whose decision to grow a moustache for another film required 8 million dollars of horribly unconvincing SFX, and another star who'd become emboldened by the massive success of her first starring role. And, on top of all that, they had to make sure the movie came in under 2 hours.
The fact that any of these guys managed to not have a nervous breakdown is remarkable, so I can totally get why they were probably incidents on set in which they were gigantic assholes to the people working under them.