Yeah, the initial velocity of the bullets would be the sum of the plane's velocity plus whatever velocity the bullets gain coming out of the barrel.

But, what apparently happened is perfectly believable (if extremely unlucky). It's not that the F-11 was faster than its bullets, it's that it flew in a way that its path intersected the path of the bullets it had fired. As soon as the bullets left the barrel they would have started slowing down, eventually ending up on a ballistic trajectory. The F-11 would have stayed at approximately the same speed, but if the pilot hit the afterburners it would have sped up considerably.

Still, the likelihood of the speeding up plane hitting the slowing down bullets is extremely low.