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The key word is "clap", which evokes the idea of a short, loud burst of sound. Rolling thunder is quieter but longer - it takes time for a sound to be "rolling".
P.S I didn't know this before writing this, but there are actually defined lengths and pitches to classify different types of thunder, which is neat! https://en.wikipedia.org/wiki/Thunder#Types
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I don't think that a thunderclap and rolling thunder are distinct phenomena. A thunderclap at Wintercote would sound like rolling thunder at Granger House, since those locations are quite far apart. (I'm not a meteorologist, but preliminary research seems to confirm my intuition: rolling thunder is essentially just the result of a thunderclap reverberating, and the rolling sound travels farther than the clap). However, I agree that the only clue that tells you who died is the phone call. The rolling thunder by itself doesn't tell you anything, but once you realize there was thunder at Wintercote, then the rolling thunder pins down the time.
