How can an outlet with only 2 wires (hot and neutral) be grounded?
The conduit is the grounding path

Note that the wiring in your case is run not using sheathed cables, but as individual wires inside a metallic conduit (aka the pipe-like stuff you see heading off to the left in your picture).  As a result, the conduit is a serviceable grounding path in its own right, connecting the receptacle grounds and boxes to the grounded panel enclosure without any need to run an extra ground wire through it.