Direct ab work can be very important for some individuals, depending upon their goals, and what weak links they may have. That said, the role of the abs is usually to stabilize the core, not produce lumbar flexion, so most variations of sit-ups, as they're usually performed, aren't the best choice of exercise.
In the video shown, the movement the movement isn't lumbar flexion, but hip flexion with the core acting isometrically. This also means the exercise will train the hip flexors to a large degree. This may be a bad thing, or a good thing, depending on whether the hip flexors are tight, for what reason. and whether the individual might benefit from stronger hip flexors (someone's who's sport involves sprinting would).
However, in the video, they're just having fun and goofing off. The exercise may or may not be part of their regular training. Also, there are many other ab exercises that could be done where it's easier to avoid lumbar flexion.