Well we all can agree that inhaling using the diaphram for the breath hold is a must correct?
Breathing from the diaphram is considered to be the most natural, healthy way to breathe at ALL times; you can do shallow breaths from diaphram as well. Hook breathing and chest breaths which are used as the first few breaths after diving are not 'natural' but neither is the extremely deep, near limit type diving for which those techniques were developed and adopted.
I know most schools seem to teach these deep breathing techniques as part of breathe up and I have read that most modern adults do not breathe from the diaphram so perhaps this is part of the logic; these coaches feel students need to be taught to breathe correctly. Luckily they also teach students to watch each other and to never dive alone as well. And as was mentioned, mild hyperventilation leads to very fast easy gains in beginners. In a supervised setting it is certainly a safer practice, moreso if the person has an untrained CO2 tolerance, in which case they will abort the hold long before they are seriously hypoxic. There are rare exceptions; some untrained individuals can effortlessly swim right into samba. If you are one of those individuals I would assume your instructor caught on quick and warned you; the one guy I met like that had taken a class thank god. If he had bought fins and headed into the ocean alone to give this freediving thing a try he would probably be dead.