The Artemis will be armed with the new Prometheus cannon. Our nebula gas miners have gathered sufficient resources to resume production of the GTW-5 Prometheus S. The S-type is based on the original design of the Prometheus cannon. With high marks for shield and hull damage, the Prometheus was our weapon of choice in the Great War.
From : The King's Gambit Command briefing
So it's explicitly said that they gathered the gas in the nebula. Given than they wouldn't be stupid enough to gather gas in a combat zone if they could do it somewhere else, it means that they don't have any sufficient argon source (we're talking canonical here, argon may be abundant in the real universe, it's not what's interesting us here) to produce Promy S in sufficient quantity to supply combat units.
Given that there are gas miners heading on over to the Colossus very shortly after the nebula itself is opened ('Into the Maelstrom,' which occurs shortly after the destruction of the Ravana), and that the design was clearly not fabricated in the span between the discovery of the Knossos and 'Into the Maelstrom', I'd say we have canonical evidence that the gas miners were intended to work anywhere
but the nebula.
I'm not saying 'you're wrong, I'm right' - after all, the gas miners gather stuff other than argon, and argon may be only found in the nebula - but as with many things in FS, I think there's plenty of wiggle room.
So, yes, you could argue that argon only comes from the nebula or from Earth. You could also argue that they didn't have effective gas mining equipment online yet, that the miners were deployed directly to the nebula once available because it was a motherlode, and that - with the nebula gone - they will now just send their newly available miners to other sources.