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.
If an aircraft starts pitching down, and it wasn't commanded by the pilot, and resetting the trim (temporarily) fixes the problem, the issue is almost certainly with the trim. If it is MCAS and a malfunctioning sensor or a failing motor that occasionally kicks on and drives the trim to its limits, the pilot response should be the same. This is what I hear Walter saying, and it makes sense. MCAS just makes it more probable to manifest.