@jack I disagree. Cliffhangers in serial storytelling always imply continuation, even when an actual continuation isn't on the cards. Ambiguous endings don't imply further stories but instead refuse to resolve specific threads in the story, leaving it up to the viewer/reader.
You could do ten more seasons of Watchmen without ever resolving the question left by this ending, if you wanted to keep the mystery. You don't need to resolve it to continue the story in that world. It is intentional ambiguity and you could choose to maintain in future stories. That usually isn't an option with cliffhangers.
This has much more in common with the ending to Sopranos than a cliffhanger.
IMHO, and all that :-)