Studies show that non-overweight or obese people with sleep apnea are extremely common and make up between 20 and 40% of all apnea sufferers:
1. https://pmc.ncbi.nlm.nih.gov/articles/PMC5181619/
2. https://www.nature.com/articles/s41440-024-01669-9
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC9130173/#:~:text=Ap...
4. https://ejo.springeropen.com/articles/10.1186/s43163-024-006...
5. https://home.liebertpub.com/publications/respiratory-care/68...
"Nasal breathing exercises" cannot overcome physiology and cannot help you if you are sleeping and have no conscious control over your muscles. Apnea is caused by physiology and while it can be made worse by weight, weight loss alone does not fix it. It may bring some people's apneas down to an acceptable level, but that is not the case for all or even most patients.
Your advice is roughly the same as telling people to skip their flu or COVID shots and hope their diet and exercise keeps them from getting sick. It's not that diet and exercise are bad for you, but they aren't the same as an exact treatment for the problem you have.