The primary source is a leaked podcast script from a podcast run by an anonymous celebrity gossip Instagram account which was posted on reddit. This is basically the journalist equivalent to using 4chan green-text as a citation. The article, which fails to verify anything, is a year and a half old and not a soul related to the production has corroborated any part of it in that time.
No official statement was made about why Cavill is no longer on The Witcher. I wouldn’t doubt that he was being difficult on set considering his love of the source material and how little the show respected it. I also wouldn’t be surprised to find out his contract just ran out and neither party was interested in renegotiating.
James Gunn has specificly said, on thar record, that Cavill wasn’t fired. The next Super Man is just going to be younger. After how badly Black Adam flopped, I’m not surprised they’re taking the DCU in a different direction.