Those astronauts maintain normal CO2 levels, and divers can experience increased CO2 buildup if they don't exhale properly, which is the opposite.

It's not made up. Plenty of people here who tried breathwork can attest to its power to bring you in strong trance-like states.

Try it yourself. Ideally in group setting, because doing this on your own (with a YouTube video or whatever) won't give you the same experience at all, and could be dangerous if you take it too far.