Hmm seems he is a retired researcher, although it doesn't mention his background. The problem is it's quite a step up from anecdotal evidence from one guy, to doing any kind of trial involving volunteers, and it would need momentum and money behind it.

It does seem there is some evidence already on breath re-training / diaphragm strengthening: https://www.europeanreview.org/wp/wp-content/uploads/4547-45...