That's not a placebo. A placebo would be learning to play some other instrument like a piano, that doesn't involve breath. I also RTFA and saw no mention of any meaningful control group. For all they know the effects would come from learning any instrument, or just going to regular classes, or...

Skinny person with OSA here and three corrective jaw surgeries from world-expert sleep surgeons. You won't believe how many GPs told me to my face that it was impossible for me to have sleep apnea because I'm thin. Then the sleep study results came in with 30 AHI (severe range).