Refute what Hermit?
SCUBA uses pressurised air, of which 21% is oxygen. When you breathe you use 4% on average. In SCUBA, your expired breath is lost as the mechanism is open cycle. You're never going to use up all the oxygen available in the tank. Most of it is expelled.
The Apollo PLSS could store over 1kg of oxygen (it varied over the missions as needed) leading to 100% O2 for the astronaut in a closed cycle environment. A human consumes 850g of O2 in a day. The PLSS units were recharged at the end of each EVA and the O2 topped up.
There's more to it but that's the gist. Anyone with a slight technical acumen can figure this out.