Assault Cannons only fire "special ammunition," which is to say, Assault Cannon rounds. Assault Cannon rounds are quite a bit more expensive than standard ammo, so they get their own slot on the ammunition price/availability chart. There is no "standard ammo" option for Assault Cannons.
So yes, with the damage listed as "as cannon," that's them explaining that the Assault Cannon rounds don't add to, subtract from, or otherwise modify the listed damage code of an Assault Cannon -- but they have a different price and availability than (every other weapon's) "standard ammo," so they have to be listed separately.