No doubt IMO it's an SSD expansion cart. PS5 default SSD will be on the MB but in additional I think there be an internal expansion users can upgrade. Like PS4 and PS3. You remove a screw and plug it in. I think possibly this way they only put maybe 500GB internal SSD to keep the console price down instead of going a full 1TB or more. The hidden cost in buying more storage.

Bite my shiny metal cockpit!