Yes, it's just a guess of course .. but while it's difficult to imagine how a planet might get so oxygen rich, I think all it needs is something producing oxygen even when everything else is oxidised. The idea of all the fuel being used up, so there aren't any fires, doesn't explain why there's so much oxygen in the first place. But it would (I think) be possible to have stable oxyegn at that percent. How it got there is more mysterious.
Yes, how it is maintained in a free state given its reactivity is perplexing. Just about everything else on the surface is going to be reactive.
Oxygen is, i think, the 2nd most abundant element on (or rather, 'in') Earth's composition, after iron. And it's similarly ubiquitous on other planets, such as Mars. But it's bound up to other stuff - and the process of oxidation is exothermic, generating heat. Separating it out again requires an equal opposite input of energy.
And if for example that energy source was solar radiation, then there'd presumably also be significant solar wind and thus alpha particles (basically hydrogen nuclei) streaming into the atmosphere, thus generating vast quantities of water...
So you'd have a hot steam atmosphere - perhaps accounting for the extreme humidities noted - but then also making the extreme atmospheric O2 levels all the more puzzling. You just can't have it floating about free w/o a big badaboom. About 15% i think is the critical limit, at which point the atmosphere will spontaneously combust. IIRC.
