I think it makes a bit more sense to train for a bit more muscle strength and muscle mass in the core for punching power. On the boxing science instagram page they always say that core mass is the biggest contributor to punching power (not sure what they mean completely, I'd guess they mean core mass correlates more to punching power more than muscle mass in the arm or legs).
Definitley true. One might be forgiven for thinking leg muscle is more important but arm muscle?? The abs are the main player in transferring leg power and connecting the whole body. I would always advise to train planks or similar calisthenics to target the abs if they are concerned with increasing punching power and technique, which are basically the same thing (imo).
On the topic of resistance bands, someone already said the resistance is minimal at the start and increases towards the end. This makes no sense in the context of an actual proper punch. You're probably working out ligaments and tendons more than anything. Power really comes from technique, which is the way you do something, and with all the combinations of bones and muscles there really is a lot of lot of ways.