From a historical perspective, this is almost correct. During Nobunaga's years before he became lord of the Oda clan, Hirate was his mentor. He was deeply troubled by the young Nobunaga's eccentricity and infamous reputation of being, quite frankly, a damn fool. Nobunaga was deeply attached to Hirate because of his role as his mentor, and when Hirate committed suicide to shock Nobunaga into acting more proper and less like the fool he was known to be, it worked like a charm. I assume the devs placed Hirate's benevolent grave at Yoshimoto's camp as a subtle way to say that Hirate is watching over Nobunaga and encouraging his success even in death.
