I’m not claiming expert status. You see in the third photo that crusty thread/string of tissue coming off the end of the “balloon”? Also, I can’t see an opening. The vent should have an outlet and despite the swelling, I think it would be visible. You can tell that better than I could... maybe it’s not visible in the photos. It really seems to be unabsorbed yolk sac. It’s likely hiding/compressing the actual vent (between navel and tip of tail).
Whether it will absorb and heal, I think you’ll have to wait and see. If you put it with the other babies, they’ll peck at it, so if you want to remove it from the brooder, put it in its own box. You can use a heating pad (wrapped in a protective bag or etc.) or seedling heat mat. (Most therapeutic heating pads have an auto shut-off after 2 hours.)