you are totally correct! I had to do a little research in order to clarify this for myself:
The colloquial name for those tractors, as I mentioned, is semi diesel (“Halbdiesel” in German - was even the name on the brochures and stuff…those tractors were started with diesel-gasoline mixtures and spark plugs, as I explained).
But, scientifically, a diesel engine is defined as a compression-ignition engine, that ignites the fuel just due to the air in the cylinder being hot from the high compression (in German called self ignition). Therefore, the hot bulb engine is not a “real” diesel engine (acc. to this definition). It depends on the hot bulb, which can be considered an ignition starter or external ignition device. AIUI, that is where the semi-diesel or pseudo-diesel name comes from.