After focusing on the Mind Palace, they realized that the Mind Palace was like an egg, and what they had been fusing with all along was its shell.
From another perspective, they are not fusing with the egg, but are digesting it as if they were a snake. Unfortunately, the shell is standing in the way of getting access to the sweet and nutritious yolk within the egg.
This is why they can only borrow the ultimate ability of the Mind Palace for now, or block access of other telepaths to the ultimate ability, and why it hasn't enhanced their power in any other way. It is also why, even though they can borrow the ultimate divine ability, they can't master it more than 0%.
Their mastery of their ultimate ability has always been stuck at 0%. They can tell now that they would be able to gain mastery of the ultimate ability and gain raw power from the Mind Palace once they fuse completely with the shell.
