First, I don't know how good F-22 really is and how good F-35 really is, and I don't want to get into this topic. But I do remember a report of last year regarding a mock fight btwn an F-22 and a Su-35 in Syria. The F-22 was allegedly radar-locked by the Su-35. Although the account is not verified by third party, I think it is quite possible because the Su-35 pilot applied a tactics that maximized Su-35's strength in the fight. In other wards, how good a weapon really is in a real war situation is not completely determined by the people who design and make it, but also determined by the people who use it. Back to the J-31 program, I think it is not only a "possible", but also a "must" that it must be able to fight and beat the F-35 when it is used by PLANAF. In general, I am not a believer of US weaponry superiority. I think Vietnam Air Force, which was trained by PLAAF during the Vietnam war, has more to say about USAF superiority in the air war in Northern Vietnam sky, if PLAAF's scores against USAF during the Korean war are not convincing.
Second, I do believe that Boeing knows more about "aerodynamics" than COMAC. But knowing aerodynamics is one thing, respecting it is another. And it is not related to your ability to walk. I don't believe COMAC the Toddle would keep upgrading engines on a fifty years old air-frame without going through due "aerodynamics" analysis. Anyone with rudimentary knowledge about aerodynamics would like to see the 737 go through a full spectrum aerodynamic analysis once new power plants, that were larger in size, more powerful in thrust, were used, but aerodynamics expert Boeing apparently disagreed. Would you fly on the 737 Max-8 after Boeing tells you its MCAS has be upgraded, thus it is airworthy?