It actually did surprise me when I heard about this feature some time ago. The reason was that Lucid said in several interviews that there were two reasons for using a trunk rather than a hatchback design: structural rigidity and better isolation of road noise. In these interviews Lucid said that no matter how good the sealing around hatchback lids, it is still a source of noise getting into the cabin. From this I had assumed that there would be a solid wall between the trunk space and the cabin in order to isolate trunk resonances.
I'm wondering if these folding seats might not be a source of the unexpected amount of tire noise that several reviewers (InsideEVs, Out of Spec Motoring, and Transport Evolved) have noted in the rear seat.
My 2015 Tesla Model S with the Executive Rear Seating option did not have folding backseats, although the rest of the Model S lineup did.