
You can build muscle doing almost anything as long as you actually go to failure or close to failure. I don't think this girl did, though, so she likely didn't stimulate much growth. Mechanical tension is the primary driver of hypertrophy and I didn't see much from where I was sitting… Anyway, even ignoring that, it is a better use of your time to do less reps and more weight. Would you rather do 4 sets of 20 reps for ~20 minutes or 2 sets of 5-8 reps for ~6 minutes?