Looking at your last picture I see those studs were put in after the flooring, which means this house was sealed up before this short wall was installed. Also looking at the first pic if this was structural you'd see 2x8's laminated. This is drywall framing to bring down the size of the door to normal indoor specs. Something held the wall up while the flooring was being installed...

If if were mine I would put temporary jack support in the doorway (threaded type) and apply light pressure. Knock that wall out. Gently lower the support and observe the above brickwork. It should not move at all. Then I'd reframe the door with fresh 2x8's to give myself a nice nailing surface for the trim when I'm done and adding more strength to the doorway.