That sail-less submarine from Jiangnan looks way too big to be attached to something like a SSN or SSBN. I think it's an experimental sub to test out various technologies and subsystems for future submarines or maybe even UUVs (a stretch but nonetheless a possibility).
If this is not a covert submarine, then it smells like a test demo for a future attack submarine.
Without any sail, this thing isn't going to go up for air which means no snorkel. So the submarine has to be charged before it goes down, so it will have to be pure battery, like pure lithium ion. Or you have a completely closed cycle AIP system. Any of these two alone should be big news with profound significance on their own, and this is one of the most underrated PLA-PLAN watching developments in the past year.
But anything bigger and man able, the nuclear power option is the most conservative and proven technological option, compared to going all Lithium Ion or closed cycle AIP. The Yuan class, which Jiangnan made a few, is said to displace 3,600 tons submerged, and that's already larger than some of the smallest nuclear submarines ever made, the Rubis class (France), which is 2,600 tons submerged, Skate class (US) at 2800+ tons, and the Alfa class (USSR) at 3,200 tons.
And even that, the smallest nuclear submarine ever made, the US NR-1, is only 400 tons displacement, and looks smaller than this sailless submarine. Even if the possibility is remote, I can't rule out the sailless sub might be nuclear.