I don't know how he came up with that.
I read 250.118 I see "a combination of the following", implying you can use any of these in combination. Maybe he is looking at something else. I suppose the option is to hook a #8 to that #12 and pull it through satisfying the 100a circuit and also covering the smaller one but I don't think it is necessary by the code I use.
250.118 Types of Equipment Grounding Conductors. The
equipment grounding conductor run with or enclosing the
circuit conductors shall be one or more or a combination of
(1) A copper, aluminum, or copper-clad aluminum conductor.
This conductor shall be solid or stranded; insulated,
covered, or bare; and in the form of a wire or a busbar of
(2) Rigid metal conduit.
(3) Intermediate metal conduit.
(4) Electrical metallic tubing.
(5) Listed flexible metal conduit