Above subsonic and supersonic true airspeeds, the airframe of the Flankers is too weak to carry such large G-loads. The early airframes, including the later J-11A and J-11B (Flanker copies), also suffered from this issue. On later models, such as the Su-27M and subsequent designs, the airframes were strengthened and are now able to withstand higher G-loads at both subsonic and supersonic speeds. So, it's not a mistake, it's actually designed like that in reality!
You can safely fly with Direct Control at a true airspeed of approximately 700 km/h. In this case, you can subject the airframe to any load without fear of the wings breaking off, this is a significant advantage in dogfights, for example. Because as long as you maintain energy, you can maneuver the Flanker at a higher angle of attack.
