I like the idea that the pact of bones/feast of bones and the tithe of bones are unrelated.
Skeletons can't be particular taste, but man flesh...
It makes more sense to me that the Mawtribes had an alliance with the Darkoath during the events of the Malign Portents "Pact of Bones". Something has broken that alliance and now the Mawtribes plan to feast on the tribes of man. So I'm guessing new army for death, box set for Mawtribes and Darkoath.