He probably was giving it throttle and slipping the clutch as he put it into 4th, in which case it would have been simply bouncing off the rev limiter for a second. Had he fully let out the clutch, the revs actually would have dropped.

Either way, it's highly unlikely that he actually went over the redline.

The 5-6 shift can be a bit tricky at high speeds because it takes more arm-force to make that shift and it's easier to accidentally turn the wheel with your other hand. Also you're concentrating on the road ahead. The last time I shifted out of 5th at redline, I only got it into neutral. I took that as a sign from god and backed off.