Flexed-arm hang won’t work as a replacement for pull-ups in the context of this particular program; maintaining a load in a static position is very different from moving that load up and down.
As a former Marine, in addition to the difference pointed out by jsteinhardt, the flexed arm hang works your arms only, not your back. Pullups require an overhand grip, whereas the flexed arm hang (and chinup) focus on your biceps.
Flexed-arm hang won’t work as a replacement for pull-ups in the context of this particular program; maintaining a load in a static position is very different from moving that load up and down.
As a former Marine, in addition to the difference pointed out by jsteinhardt, the flexed arm hang works your arms only, not your back. Pullups require an overhand grip, whereas the flexed arm hang (and chinup) focus on your biceps.