Some did and some did not !! Not sure about Arkansas at all but have looked a bit in that area !! Caddo from our area had little decoration if any on most of their pots !! Most seemed to be simple utilitarian !! Could be different area wide !!
I am just saying from here, and there was a large Caddo population, they might do a little paint on a pot and a small amount of engraving but generally it was minimal !! The Arkansas Caddo were a bit different culturally I think,, but that is only my opinion !!
Ray,, that last link you gave down at the bottom even questioned the seed jar theory !! And gave logical reasons due to the propereties of the pots themselves why they doubted that was their intended use !!