As a general rule, the answer to the question "Why is this fitness influencer doing this exercise or workout in this unusual way?" is either one or both of two common explanations:
- They have no idea what they're doing.
- They're deliberately trying to overcomplicate things in order to give the impression that not only was an incredibly complicated solution necessary, but that they're so clever that only they could come up with it.
In this particular video, given that he makes some egregious errors just in the first couple of minutes (such as confusing spinal flexion and extension, and claiming that tight glutes and weak glutes have identical effects on the lower back), it certainly looks like explanation #1 is applicable to some degree here.
That said, his routine is still very likely to help with low back pain, just because pretty much any form of exercise you can tolerate will help with low back pain.
I figured that if the goal is to gain strength in a weak area like the lower back, then the approach is usually low reps with like 3 sets in sequence, before moving to the next exercise.
If the goal is to develop maximal (i.e. 1RM) strength, yes. But that's unlikely the goal in dealing with back pain.
So did the instructor decide on circuit training simply to reduce the time of the workout? There is less rest with circuit training.
This is more of a goal of circuit training where the circuit is structured to avoid working the same muscle groups for two exercises in a row, so that the muscles used in one station get a rest at the next station. Whereas if the muscle groups used in the various exercises overlap, that suggests that the goal is more building fatigue with low resistance, rather than saving time.
One explanation for why a circuit structure would be appropriate in this case is because the video aims to provide bodyweight exercises only. For most people there are no bodyweight exercises that will fatigue the lower back in a low number of reps, hence choosing a circuit workout allows a lot of reps to be done without it getting too monotonous, and also broadens the range of muscles that are being worked.