The thing is the checklist says in a runaway stabilizer scenario, it will trim continously, but MCAS does not apply trim continously. Therefore it does not exhibit itself as a "normal" trim runaway. Checklist: https://i.imgur.com/K7El4K4.jpg
Hence, they might not have thought it was a trim runaway.
Not every emergency manifests as a textbook case. It isn't too farfetched to imagine an intermittent short running the trim motor on an intermittent basis rather than continuously. I'd still shut off the trim motor as a possible remedy, but I'm not an airliner pilot, just a guy who flys for $100 hamburgers.
Hence, they might not have thought it was a trim runaway.