Who is this guy being randomly quoted?
J-20 maximum speed is Mach 2.0, which is all you really need. It's the same as Su-57 and F-22 maximum speed. In fact F-22 at Mach 2.0 is only using 36% afterburner (118% throttle, with 100% being full dry thrust and 150% being full afterburner), but it's normally limited to that speed because any faster will just be a maintenance nightmare for the skin. J-20's DSI might only be stable up to that point before you run into conditions like inlet buzz, especially since it doesn't have the bypass louvres like on the F-22 (neither does the Su-57, actually). But again it doesn't matter because being faster than Mach 2 is never used operationally unless you're a MiG-31 and even then it's questionable in a modern environment when the thermal signature will light you up like a Christmas tree.
It's Xi Yazhou, but what he is saying is that 央视, basically central state/national media of China, stated that the top speed of the J-20 is 2.8 mach.
While there have been cases of incorrect statements and reporting from 央视, cuz people can make mistakes, but it is also often take as an official source including confirmation of stuff.
As for this case, it could very much be true that the top speed is 2.8 mach, but even then it doesn't mean that much, because we really have no idea of how applicable or useful it would be for combat (just because somethings 'top speed, max range' etc. is X number, doesn't mean that it will automatically be really relevant for combat).
