Why? Different fora have different standards of proof. For example, in civil cases (even in America) the standard of proof is 'preponderance of evidence', not 'innocent until proven guilty'.
Why should internet comments follow criminal law, and not eg civil law, or some other standard?
Those are not the only options, those are the two extremes of a spectrum. Most people fall in the middle with something like "assume people are innocent unless you see convincing evidence of guilt". This is a reasonable philosophy unless you have power over someone, in which case proof is much more important.
Because the report only contains statements of fact related to the police report and the police interaction.
There's no actual confirmation in that report that her arm was actually broken or that she was actually beaten. There's no medical examination that happened here that is cited.
That would still be required in a civil trial with preponderance of evidence. What if she was on drugs and did it to herself? (Not saying that's what happened). We don't know what happened from this document and that has nothing to do with this charge or his appeal.
The options are you assume people are innocent unless proven guilty, or guilty unless proven innocent.
Going through life treating everyone as guilty until proven innocent sounds like an exhausting and negative way to treat everyone, and harms more people overall.