Seems to me that they arent confident enough in the WS-10C.
J-20 WS-10 pilots in Cangzhou and Anshan have switched to the new type for under one year. You seriously want them to go fly in a 15 aircraft formation when they have under 100 hours on the aircraft?
If they didn’t have confidence in the WS-10C then they wouldn’t have built so many airframes in Chengdu (they suspended production of J-11B for that reason), let alone allow two WS-10 equipped aircraft to perform a flyby over the PLAAF academy half a month earlier.