Would a truthful Emperor still give a usable account?

Could the Emperor answer usefully after confinement to the throne if his mental condition had deteriorated?

The proposed answer assumes that the interview rule guarantees objective truth, despite the objection about his ability to explain events.

A second proposed interview places the Emperor after his confinement to the throne, when he knows what happened to his plans. That timing would allow questions about the outcome as well as his original intentions. Another participant questions whether his mental condition would permit a useful answer. The reply interprets the scenario's guarantee of truthful answers as a guarantee of objective truth. The fictional exercise therefore depends on whether its rule supplies reliable knowledge or merely prevents a witness from lying.