  :the derivation seems fine. IIRC, at least one graduate stat mech text gives a presentation along this line. if the point is that the microcanonical ensemble equal-probability assumption is not needed, it's not used in the article. the sentence in the introducion is meant to indicate an alternative derivation. the fact that the system is coupled to a reservoir is same as saying it's in equilibrium, which is reasonable. and indeed one can define a Gibbs ensemble to be the probability measure induced by the Gibbs distribution. this is hinted at in the article quantum statistical mechanics, and is standard in mathematical literature. in fact, i've never seen it defined otherwise there.  