Yeah yuri manga struggles to get readers that turn into buyers. Better bully one of the best scanlators on the web so that folks don't even get as far as reading, that'll help.
Were there other letters from a different person? There are no threats whatsoever in the one you posted.
Sending a physical letter to someone's home address based on their online activity in their private capacity is overstepping boundaries and I can understand people being creeped out by it. That's still a far cry from the serious allegations you are making in your comment.
It's not clear if there were actual threats or if you are just engaged in hyperbole.