My memory is rusty and the link doesn't corroborate your figure from what I could see. What I remember is that methane is more potent a greenhouse gas than O2 but it has a significantly shorter half-life in the atmosphere.
Methane has a half-life of < 10 years (7, I think) but a GWP of 25. The primary concern of atmospheric methane is, then, the possibility of a quick build-up that increases CO2 emission from natural sources; there is quite a bit of CO2 stored in bogs that only a bit of methane will release. It's the long-term side-effects of atmospheric methane that become long-term concerns, rather than the methane itself.
