another possibility is that the upper a-arm actually controls 3 dof rather than the typical 2. The outboard end seems long enough longitudinally to plausibly contain 2 ball-joint connections to the corner instead of 1 like a conventional wishbone. This would result in the arm constraining 3 dof. The rear leg seems thick enough longitudinally to maybe support this arrangement (similar to the Merc front suspension lower wishbone).
There is a rule that says the suspension can't be over or under constrained, but it's not clear what that means in practice. If there were two ball joints on the outboard end of the upper a-arm, then the suspension would not in fact be over or under constrained.
It's fun to not know!
Least interesting possibility is that we are missing a link due to angle of pictures.
EDIT: the 6th dof is being held in place by Ron Dennis' sheer force of will!