You surmised correctly. The one buried in the courtyard is Kirigiri Kyōko’s maternal grandfather, his name was Tōhachirō Uzuchi, he was a talented old man who had a mastery in the 8 different traditional Japanese arts: Kendō, Jūdō, Kyūdō, Aikidō, Iaido, Naginata, and Shodō, and was also a loyal supporter of the Kirigiri family."

The self-defense techniques that Kirigiri Kyōko learned might have been taught to her by Tōhachirō-san. The actions Shinsen used when he faked his identity should have imitated Tōhachirō-san's style. The man sitting in the driver's seat seems to have the same martial arts. Perhaps he practiced self-defense in the past when he was learning detective skills.