There was a point where lots of people doubted Fishers allegations against Whedon, "where's the evidence?" they said. Then others who worked with Whedon spoke out about his past behaviour. Charisma Carpenter publicly supported Fisher and talked about Whedons behaviour and the stars of Buffy came forward to back her up.
A pattern of behaviour confirmed by multiple corroborating witnesses, a chain reaction started by Fisher revealing what people in power in the companies are able to get away with. Things that were kept secret for years because others were choosing to enable and support Whedon.
I don't see many people doubting the allegations against Whedon anymore.
So why the doubt when it comes to Johns? If we accept what Fisher said about Whedon is the truth, then we have to consider who let him get away with it, who enabled him? Johns was there, others have spoken out about his own behaviour, and Fisher seems to have been proven right about Whedon.
And if not Johns, why would Fisher give whomever was actually enabling Whedon a free pass and target an innocent instead?
It has been interesting seeing the conversation change from "Fisher is a liar" to "Yes maybe Fisher was telling the truth about Whedon, but everything else he says is a lie."