It seems like it was written by a politician. Someone who maybe can't publicly endorse the db. For some reason I suspect Elisif, even though the courier received the letter from a male. Using a servant to deliver a letter to a courier is just the sort of thing you would do if you're in a position of power. Especially if you feel the need to distance yourself and stay anonymous. She would also be in position of needing a powerful ally, and unlike ulfric she doesn't die, though she would still need some sort of motivation to continue sending letters if you side with the stormcloaks. As to how she gathers her knowledge I can't say.