I think the author may have pulled the 9 month figure from Wikipedia. The Perseverance rover trip is about 6.5 months. The Curiosity rover took a little over 8 months. The Insight lander took a bit over was 6.5 months or so. I am guessing the time travelled is dependent on nailing the launch window and the launch vehicle required to get there..
By definition, however, a Hohmann transfer is a minimum energy transfer. Any variation in timeframe, either longer or shorter, is no longer the minimum energy. But I have a history for nitpicking scientific inaccuracies. Especially in movies. You don’t want to hear them.