Why is hydrogen formation in serpentinisation an exothermic reaction?
Under normal circumstances your suggestion might be correct, but bear in mind that the serpentinization reaction takes place under very negative Eh, and very high pH conditions (pH 12 or more). This abnormal combination is right on the boundary of water-hydrogen stability conditions. In hyperalkaline springs, which result from the reaction water + peridotite = serpentine, there is hardly a free molecule of oxygen left in solution.