He didn’t have to be silent, but nothing forced him to put his neck out and for the club to declare they believe him not guilty of the acts he was charged with.
He could easily have hid behind due process, the courts and an alternative explanation from the family, without adding that conclusion, not least when the same statement was already confirming MG would not be reintegrated with the squad and would instead attempt to rebuild his career away from MU.
However you spin it, Arnold absolutely did say much more than he had to there.
No, he couldn't easily have hid behind anything. It was reported everywhere that the club was ready to reintroduce him to the squad. When that leaked and was met with backlash, it was reported that they would be consulting with the women's team, sponsors and supporter groups before reaching a final decision. This was again met with backlash, because it would definitely lead to shitheads blaming the women's team if they outcome was Greenwood goes.
He's denied it, but he also knows perfectly well how little that denial is worth in the face of all the reports of what their actual plan was. So he has no choice other than to say that he believes Greenwood is innocent, because that's what does the least damage to his and United's reputation. And it'll just have to look a bit weird that he does so while simultaneously announcing that Greenwood doesn't have a future at the club.
They also said they had evidence that essentially exonerated him didn't they?
No, they said they were given alternative explanations for the audio and pictures, and that seems to be it in terms of "evidence."
Arnold also said the audio was a snippet of a longer recording, but in a way that makes it incredibly obvious that there's nothing on it that actually helps Greenwood. But he correctly predicted that the mere mention of a longer recording, combined with the club confidently stating that they believe him innocent, would lead a lot of people to assume that there's something on it that exonerates Greenwood.