In
Powell v. McCormack, Powell was directly harmed by the complained of actions - he had a particularized and direct injury and, as such, no party contested his standing.
Nowhere in
Powell v. McCormack does the Supreme Court disavow the standing requirement. It didn't address standing - quite simply because standing was not contested.
I give you an example where the text of the Constitution was breached, and no-one challenged on grounds of Standing, and you say it is not enough. It is exactly on point, and Keyes and the rest, in fact every Citizen, has standing in this particular case, because the text of the Constitution itself is being dis-regarded.