I don’t think they explained what evidence they didn’t have, just that they didn’t have all of it. I would be very surprised if they didn’t have the Police statements but have you seen something that expressly says that they didn’t somewhere?
In any event, if they had less evidence, it’s even more surprising that Arnold would be advised to say that he, personally, was satisfied Greenwood was innocent of the acts he was charged with. Again, there was no need whatsoever for the club to include that wording, and it must mean that they feel very strongly that it is true.
In any event, unless or until someone explains all the background, which is unlikely to happen, we can only speculate. I do find it somewhat astonishing that the likes of Holt can accuse United of misogyny for carrying out a seemingly thorough investigation just because they reached the opposite conclusion of that he would have liked them to reach, without having actually seen that investigation.