Hard to be certain, but if I were to guess, I'd explain the reasoning like this:
Luther had done something unprecedented. There was one Catholic (universal) Church, who claimed that they alone held the keys to salvation. But Luther had dared to claim that salvation could be found outside the authority of the Catholic Church.
Obviously this gained him a great deal of notoriety, and some amount of power as well, as he ended up with followers and disciples. And he could easily have imagined that this would end up happening when he started out. This leaves the lingering question, did he do it all for the glory? Or did he genuinely do it because he believed it was the right thing to do?
If he was a fraud, his disciples are in big trouble and they'd better get back on the path as quickly as they can, and go back to the Catholic Church and seek forgiveness. Otherwise, they end up damned for eternity! So they ask him on his deathbed, when he knows his time is over and he has no more reason to lie, (if it was a lie) whether the whole thing was genuine. And he affirmed, with no reason to lie, that yes, it was.