To reduce hydrogen, something needs to be oxidized, but it doesn't need to be oxygen. E.g. you could get more metal oxides. Not my field exactly, so I don't kno

To reduce hydrogen, something needs to be oxidized, but it doesn't need to be oxygen. E.g. you could get more metal oxides. Not my field exactly, so I don't know either relative abundances of different stuff in early oceans or the exact ranking of which would be most readily oxidized, but that could explain the discrepancy. Edit: rereading the passage you quoted does make me think that metal oxides are the important factor here.