Just because the characters thought it was a map to Thrawn they were looking for and it actually being a map to where Thrawn was doesn't mean that it was created with that intended purpose.

It seemed pretty heavily implied that Lady Morgan had supposed what happens to Thrawn and knew of the existence of this device that would let her travel to where she believed Thrawn was.

Disney has taken a lot of creative license with Star Wars cannon, and has been very unapologetic about it. At first I thought this was another example of fuck the story we want to sell toys, the surprise tie in to the final scene of Rebels was well done and gave me some serious nostalgia vibes.