With respect, long-range transmat/teleport was established in Doctor Who... back in the 1970s, I believe. And, the Doctor has a box bigger on the inside that goes anywhere in time and space, and you're worried about just moving people around?
Um, Doctor Who isn't about telling you the exact history of its monsters, unless that history is plot relevant.
Be that as it may, it is important to note that "breathing" means different things for different organisms. For you and I, "breathing" is taking in an oxidizing agent that we then use to burn fuel. For plants, "breathing" is a process where they bring in the bulk of their building material (carbon) - plants build themselves out of thin air!
Chemically, carbon dioxide doesn't work as an oxidizing agent - it is already oxidized, obviously, thus the name. So, it must be that these creatures are acting more like plants - they are taking in carbon dioxide for the carbon, and they release the oxygen as a by-product.
With respect, the show is talking about efforts to change how global society operates and a massive scale. That's not a thing you talk about once, and never mention again. That's a message that needs to be repeated, over and over, to keep people's mind on the target, even if they are on-board with the notion.
If you don't want to be given an occasional moral... Doctor Who really isn't the right show to watch.