"but there was no cargo fire alarm"....entirely correct, because the cargo holds have/use smoke detectors, as opposed to overheat detectors on APU and wing/strut ducting etc.
I'm not saying it won't happen. I'm sure there are materials in the sidewalls which would generate smoke at those kinds of temperatures... dust, paint, plastics, paper items, etc. You'd be surprised what slips past the floor decompression panels on some aircraft.
Smoke will eventually find a way past the panelling.