Does that mean that the pilot can pull an additional 1.5 MAP over the determined MAP limits when operating with full carb heat?
Yep, easy to understand and delivered in the practical direct manner, as utilized by many clever yanks.
Make the air hotter (and less dense) you need to get more in there for the same power.
I would clarify it thus:-
'you need more "volume or air if it is at a higher temperature to get the same weight of air" in there'--