After all? Since when was it believed that Mars once had an oxygen-rich atmosphere?

This "refutation" is the first time I've heard it seriously suggested. There's a reason an oxygen-rich atmosphere is considered a bio-signature: it's so volatile that life continuously producing more free oxygen for a very long time is one of the only explanations for how it could exist in the first place. And last I heard the idea that Mars ever had life, even without photosynthesis to produce oxygen, is still very much under discussion, with minimal evidence in either direction (an "No" being the default answer)