The investigation was started by a non-criminal who made a prompt report of the crime. They went back through their old police reports and found another report of the same crime that lined up with his police car's GPS tracker, and then went through his criminal record lookups and his police car's GPS records and found the rest of the accusers. So the initial accusation was more legitimate than most, they found they already had more evidence without seeking out testimony from the public, and the other accusers were found based on their records of the accused's own actions.

If it was just "criminals #metoo police officer for arresting them" then you'd be right to be suspicious, but they do have a more solid case against him than that.