If the manufacturer states the SB to be mandatory then non-compliance would make the aircraft un-airworthy correct?
Essentially a mandatory SB and a AD have the same effect,
difference being one is issued by the manufacturer and the other by Aviation Authorities.
If the aircraft is operated under 91.409(f)(3) for it's inspection program (which is an option that applies to large airplanes that are turbine powered and turbine powered rotorcraft) than a Mandatory SB issued by the manufacturer must be complied with.
In the case of an export, however, the matter of a manufacturer's election to include the word "mandatory" is irrelevant, and the requirement for compliance varies, as well with the destination, and sometimes with the origin. This is not the case for an import.