It sounds like Mojang threw £20k at the author without understanding or articulating what that payment entitled them to.
At a later point they realised they didn't own the actual work itself and tried to strong-arm the author into giving them that ownership for effectively nothing.
At the time Mojang appears to believe that this could be an issue during due diligence. With the amount of money on the table, even offering a life changing sum to the author would have had a nearly imperceptible affect on the overall balance of the acquisition.