Likely what they mean is that J-20 pilots have come out of test pilot school and been flying J-10C and J-16 as well,,, its always good to expose yourself to as many different aircraft types as possible to give you a well rounded experience bank.
Test pilots encounter all sorts of weird anomalies as they bring new aircraft up to speed..
I'm NOT sure that China allows civilian test pilots or factory test pilots?? anybody know??
LockMart and others usually employ ex-Military Test pilots for factory work,, the Paris Airshow F-35A demo was flown by a LockMart pilot, after the airplane was delivered to France by Air Force Pilots.
anyway Hendrik, they are expressing that these guys can jump out of one, and into the other as a matter of routine,,,, run your respective checklists and "kick the tires and light the fires"! they are ready to rock and roll...
but not necessarily high time combat pilots, a "whole nother smoke"!