The Ministry of Magnates was written by Dumbledore. He wrote the chapter about himself to prevent speculation that he might have written it, to slander him in the way he wants his enemies to think of him (doddering and out of touch, so they underestimate him—rather than, for instance, secretly connected to Grindelwald or deeply manipulative, which would be more disruptive to his political goals), and to deepen Snape’s cover by making it appear that Dumbledore’s trust in him is a mistake.