True enough, but for electrolysis you'd need some sort of battery or dynamo. I guess it's *possible* to have a very large dynamo reacting with the water and splitting it, but nothing like this has ever been observed; the only known example is the rotatiing liquid metal in the earth's core (or simliar things in the gas giants). Also, I'm not a chemist, but it seems unlikely that the hydrogen that's released wouldn't recombine; an atmosphere with 20% O2 and 40% H2 doesn't seem realistic to me somehow.
The point being, if a planet is observed with an atmosphere that's way out of chemical equilibrium, there has to be a source of energy and a mechanism to keep it from reaching equilibrium. In the absence of other exotic processes, like intense magnetic fields or radiation, life is the most likely explanation.