You’re quite right. the manual says…
Company Policy on Day Old Chicks and Turkey Poults
These are packed in pre-prepared crates, stacked and ready for loading. The birds generate a lot of warmth and water in such concentrations and the packs are designed to minimise heat loss yet allow sufficient air circulation for their well-being. The flight time should be limited to 5 hours.
So I think that the significance of that paragraph is that they can go,in the unheated hold (as they generate a lot of heat) whilst live animals have to go in the small heated hold so from the airlines point of view different levels of care would be required. You’d have to,ask a poultry farmer what’s the difference between a one day and two day old poult.