He does look little stiff and could use little more flex on his legs, but is it because he is riding on very flat run?
On the video it was hard to tell what kind of run it was. The terrain looks very flat and some part of it looks like transition run. I don't ride transitions runs with flex on my knees (some but barely any). Is this you pushing yourself or are you leisurely riding? You should post something where you are pushing yourself (your weakness would be more noticeable)