Charcoal is also used as natural deodorant used in freezer and ice boxes, so I think it breathes by itself absorbing gaseous impurities and flush out some gas as well but I don't think it is very harmful. It is carbon and I think it doesn't do any harm to orchid roots since this element is also considered beneficial to plants. Whatever bad stuff it emits unless, I think the charcoal do it moderately and I don't think it is harmful. Maybe they think charcoal breathe out methane gas but with the amount of charcoal employed in pots, small amount of methane seems beneficial to plants.