You're absolutely right. 'Close Your Mouth' by Patrick McKeown was revolutionary for me. Buteyko breathing is the real deal, unfortunately it is largely unknown in the modern American medical community.
This makes me wonder if sleep bruxism and sleep apnea are mutually exclusive.
What makes you wonder that?
Because with bruxism you typically have your mouth closed (I see no other way to grind your teeth).
I don't seem to be able to simultaneously breathe through my mouth and grind my teeth. They could still happen during different parts of the sleep cycle, though. Perhaps grinding during light sleep and mouth breathing during deep sleep?