Obligating the incapable would mean demanding an act when the relevant ability or necessary means are unavailable. The argument does not concede that someone who is not presently performing an act is therefore incapable in this sense.
For example, bodily ability and the applicable conditions matter for the obligation of prayer. The command need not first become binding at the exact instant the person begins praying.
The distinction is between means available before action and the determinate capacity accompanying a particular action. Confusing these yields the mistaken impression that obligation and concurrence of capacity contradict one another.