Since the placement rules say to treat it like a character... I'd say, "yes, it counts as having been filled in by rank and file troops for the entire base of the cauldron". This is even stated on page 98 of the BRB under "different sized bases" Since the chariot base fits nicely in with 20mm base footprints then you should be good to go.

Nonetheless, I agree that it's poorly worded and leaves too much room for misinterpretation.