It wasn't a trial, it was a hearing under RFL Operational Rules. The crux of the matter being that Charnley was more of an "impressive and credible witness" than McGuire was. We can only speculate as to how they arrived at this conclusion, that tipped the balance of probabilities towards Charnley's claim. What we do know is that McGuire has already been charged with the same offence, after the preseason match with Leigh that was, IIRC, witnessed by match officials Marcus Griffiths. Amone of Leigh was cleared of a similar charge made against him, as only McGuire made an official allegation. Matt Dufty witnessed the comments made by Amone, but didn't want to make an official allegation after the match had finished.
I'm not defending McGuire, but he seems to have been treated unfairly in comparison to Amone. The club have now little option but to terminate McGuire's contract.