My health visitor told me to put my 7 week old on hungry baby milk because he was only going 1 1/2 to 2 hours between feeds and was having 5 oz a time but if you are going to give it to them only give them one milk, mixing formulas even if it's the same brand can cause tummy upset.
If hungry baby milk is only for babies who are draining 8/9oz bottles why can they have it from birth? my understanding is they don't have 8/9oz until they are 4/5 months old??
Babies are all different i guess some young babies could drain that amount but yes i suppose mostly it is 3/4/5 months so you would swap to hungry baby to try and hold off weaning, as for why it says from birth i dont know but you wouldnt put a newborn straight onto it yet saying from birth technically you could but like rusks say all ages on them or 4 months plus yet contain gluten which is a no no before 6 months so just because packaging says something doesnt mean its always the best thing.
This is just my opinion and i wouldnt swap a baby to hungry milk at a few weeks old in an attempt to get them to sleep longer. (I mean like OP's friend not refering to you)