Two words to describe why it doesn't make sense to discount a plant as "non-invasive" just because it doesn't produce seed: Vegetative Reproduction. This new clone of fiveleaf akebia (Akebia quinata), also known as "chocolate vine," has started its new life about three feet below a small patch of its parent plant. There are many new clones along the bottom of this cement wall, and none of them were planted.
I can easily imagine this species spreading from clippings in yard waste. And hey, this is on Nantucket - a big wind could easily break off new pieces and spread them over the island. So who needs seed?