Yes, some of those dealerships (but not all) are probably organized as corporations, but most people (and clearly the OP with his reference to that documentary) mean it to refer to joint stock corporations.
A union of small businesses banding together and throwing their weight around to protect their interests honestly sounds pretty analogous to a "corporation" doing the same.
It's like an amoeba throwing itself around or a small multi-cell organism doing the same. The details might differ but the effect from the outside is pretty similar.
