Is this a ground wire in the ceiling light canopy

The instructions for the fixture are only correct for a metal box. If a metal box was used, the box itself would (should) be grounded. The bracket that holds the light would then be connected to the box, which would make the bracket grounded. Finally the ground wire from the fixture would attach to the bracket, grounding the fixture.

In the case of a plastic box, the box is not grounded. All ground connections must be made by connecting the ground wires together. So in your situation you are correct, you'll connect the ground from the supply to the ground from the fixture using a twist-on connector (or other approved connection).

If you cannot route a ground wire (with continuity back to the breaker box)(to be preferred), you can supply the lighting with power from the LOAD side of a GFCI. This is why there is a "No equipment ground" sticker in the GFCI packaging, which you should put on the outlet (if not hardwired) or on the under-cabinet lighting.

This would not create a ground, but it makes a safer alternative for a fixture within reach of a water supply (6 ft in IRC code).