I don't know if the so-called WS-13E is a copy of the RD-33 or a completely different design. But I'm guessing they are similar. I also know that the RD-33 features a modular design.
The Klimov website confirms this.
http://www.klimov.ru/en/production/aircraft/RD-33-family/
And if the modular design is anything like the Pratt & Whitney F100, the augmentor/nozzle module should be fully removable and interchangeable with other modules.
If my theory is correct, that means the RD-33/93 lookalike nozzle you see now is nothing more than a module used to expedite testing of the early pre-production aircraft. China can easily snap-on/snap-off the RD-33/93 nozzle module and insert the stealth nozzle module at any time. We've already seen the stealth nozzle in models. It is not false advertisement.
Furthermore, the fact that the engine has gone from smoky to smokeless tells me that the engine core is now different from the original 31001 prototype. Black smoke comes from the combustion chamber inside the engine core. The combustion chambers (for whatever reason) aren’t able to burn off all of the fuel efficiently, so some particles of the fuel are vented out the engine, which materializes in the form of characteristic black smoke.
You can argue that the new RD-33MK is smokeless. That's true. But I can argue that you have no evidence Russia has sold the RD-33MK to China.
The B-52 is notorious for black smoke. My point is that the black smoke is coming from the engine core.
Lastly, it would make no sense for SAC to make such drastic changes to the aircraft if there were no customer for the FC-31. The FC-31 has made the same jump the J-20 did when it went from the 200X prototypes to the 201X pre-production aircraft. SAC spent a lot of money to do this, and the design is now finalized.
Also that "decreased space between engines" could mean the engine bays have changed.