That's not an answer to my question at all. I'm asking about the comments and attitudes that seem to treat mouth breathing as bad and wondering where they come from. I'm (relatively) sure that a book published in 2020 isn't the root cause.
As for the current mouth vs nose and belly vs chest breathing, it really feels like a fad/cult. There seem to be a constant stream of very small "studies" with questionable methodologies with just enough results to warrant further investigation.