Muppitz is specific, the buzz only happens when the capo is used. Therefore the fault is not that the nut is too low.

If the buzz happens every time the capo is used, it is unlikely to be a capo problem because a capo will go on slightly differently every time, so sometimes the buzz would be worse than others. You may say that if the buzz is not there if the bottom string is fingered, it must be a capo problem, but if you are fingering the string it is not left to ring on its own and the fingering will probably prevent the buzz happening.

The buzz is not stated to be limited to certain capo positions, therefore it occurs all the way up the neck.

Therefore the buzz is caused by the neck progression being insufficient and/or the bass side of the saddle being set too low. necks and saddles get adjusted this way by shops if nuts are too high, because its easier to tweak the truss rod or saddle than to cut nut slots with the necessary precision.

Hence my diagnosis above.