As someone who used to 'self-medicate'[1] my (then-undiagnosed) ADHD with breathing techniques and Yoga – mostly Hatha and Kundalini Yoga – for 15 years I am - personally - very skeptic of these claims.
Don't get me wrong, I still love breathing techniques and Yoga and practice both on a daily basis.
But I DON'T see how stimulating your nervous system this way will do what ADHD medication actually does: Inhibit dopamine/norephedrine reuptake / increase dopamine/norephedrine secretion.
In other words, in my n-equals-one world, the difference the right medication has made in managing ADHD has been an order of magnitude greater compared to regular, intense and consistent breathing, Yoga and meditation.