Wait, so if its still speculation how can you count it as canon? Also, why wouldn't Jasprose be referring to Terezi? From what we've seen Karkat hasn't been obsessive and controlling over Dave, so how can that be referring to him?

“Desired” is not filled. Jasprose was talking about two separate people. Referring to Terezi when speaking of Karkat’s old desired matespriteship and to someone else when discussing this fulfillment of the quadrant.