Waffle and Ambiguity indeed. It is near impossible to gain a clear, unambiguous position. Your take on it doesn't make it what is required by law, nor does mine. However, CASA have advised me, as the registered operator of the aircraft responsible for the continued airworthiness of that aircraft, that 'You are responsible for assessing each service bulletin to determine whether it should be followed or adopted'. I am almost as frustrated by this as I am by watching the Lions.
It all just strikes me as CASA getting the maintenance shops to do their dirty work for them. If they believe that magnetos MUST be overhauled every 4 yrs (only bendix ones by the way) because of a risk to airworthiness and safety, an AD should be issued, as compliance is then mandatory. In any case AD ENG 4 applies to 'engine and components necessary for the operation of the engine', so does that not apply to magnetos??