Should my light fixture be grounded through the box?
Your wiring method is conduit.  Your box is grounded through the conduit.
The conduit is used as the grounding method.
You can see the locknut on the conduit in the box.
The other clue is colors, like the purple, in use meaning individual wire pulls through a raceway.  Actually all the wires are labeled, which is another indication that individual wires were used.
If non-metallic conduit was used, there would still need to be a grounding wire to the box, but you can see it’s metallic.