A consulting friend of mine told me a story of how he confronted a no-decision manager with his lack of decision-making.
He was a consultant and discovered that his client was a no-decision manager. His duty, he thought, was to inform him of his no-decision status. He decided to bring up the subject, if I recall correctly at an informal lunch the two of them had agreed on.
When my consultant friend brought up the fact that he never made decisions, the no-decision manager in, question immediately went into a tirade of uncontrollable anger and indignation. He refused to accept that he never made decisions. He was a senior manager in the organisation and it was just not possible that he had got to this position without being an excellent manager. And everyone knows that all excellent managers make decisions.
This I have found is the standard way no-decision managers behave when they are confronted by someone accusing them of lack of decision-making.