His legs are not completely locked, so straight legged deadlift is not 100% accurate. The important distinction is between this and the conventional deadlift, however, not the position of his knees.
I wouldn’t recommend locking or hyper-extending your knees during this movement in any case. A “soft lock” is enough to give your hamstrings the benefit of the movement.
As to the movement, it is mainly for the hamstrings. I don’t do them off boxes as high as the guy in the video, but I do use a box. I also move quite a bit slower, but to each his own.
I’ll be honest: I don’t do these that often, mainly because my hamstrings get a lot of work with all of the kettlebell swings I do, and also the work I put in swing the Stronger Grip Plateau Buster.
Lift Heavy and be happy!