I know a lawyer at Roper-Greyell. The firm specializes in defending/protecting companies around employment law. They probably also consult on best practices for companies to avoid suits.
They are a law firm, but they are LMG's law firm. Note that we're reading what LMG wants to share about the report, not the report itself.
It's a legal strategy to say, "Investigators didn't find anything seriously wrong, and we're considering suing you for defamation"
A lot of the other replies to your comment read between the lines in some insightful ways. I would just like to also add that "claims of bullying and harassment were not substantiated" means that the investigation didn't find evidence. Not that the bullying/ harassment didn't occur. It is kind of meaningless without knowing what information was looked at. What sorts of substantiation could reasonably be expected from the investigation?
It's possible that the person making allegations has evidence that the investigators didn't.