Lisan Universe indeed accepted giving Garisan their Werdlem Treasures... but not all of them. They reached an agreement that only the Werdlem Hexahedron would be given to Garisan.
As one can imagine, Garisan didn't want only one. But, in the end, it was still a Werdlem Empire Treasure. Something that they couldn't simply obtain any time they wanted. Lisan was willing to take their chances and let Garisan leave rather than give up all their Werdlem Treasures.
The reason was pretty simple. Not only could they not use the mass teleportation the twins used before, but they were too weak after the war. If they gave everything to Garisan, Garisan could easily turn against them. At that moment, there would be nothing else that could be done.
