If you do not have a ground wire installed with the other wires do not hook up the ground wire without knowing how the wires were carried to the box. If the house is wired with flexible metallic tubing (flex) your grounding the fixture can create an extremely hazardous situation. Older homes are sometimes wired this way. I have run into this situation a lot in my field work. Untrained electricians think any ground is a safe ground. This is not true. Grounding was required starting in 1962.
What the heck is that suppose to mean.
Is he saying that you shouldn't ground a metallic box (i.e. run a EGC from the panel or another EGC at another box) that has MC cable
flexible metallic tubing (flex)
going to it if it didn't carry a EGC with it because if it was in contact with combustibles (e.g. studs, lathe & plaster, etc.) and the box became shorted it could start a fire.