Not necessarily too modern. Scientists used to believe things on Earth burned because of a mythical substance called philogesten (or something similar) existed within matter which had a tendency to spontaneously combust. No knowledge of oxygen at the time, so the plausible-sounding theory was utterly bogus.
I take it Phalanos is an alien world with an unbreathable atmosphere. The "magic" masks are ancient aqualung or rebreather equivalents. Assuming it has an atmosphere, are there critters there, and can they endure the atmosphere and gravity of Glorantha?