It is, yes, at least the first season. They spliced together existing footage with some minor edits as necessary (e.g. lipsyncing, position edits) and dubbed it to make something new. That kept the show cheap since the animation costs were almost nothing.

It wasn't the only Cartoon Network/Adult Swim series to use that approach either: Space Ghost Coast-to-Coast used a lot of recycled animation from the original Space Ghost, though mostly in the form of individual characters edited over/into new backgrounds and animations (e.g. the set of the show).