Correct. If the part requires cutting or polishing to fit, then it isn't Ok, unless it is done at the factory.
Thus the G17 extended mag release, which is a drop in part, can be fitted aftermarket. The SP01 competition hammer, which may require gunsmithing, is only legal if it is a drop in fit or if it came fitted from the factory.
I see this as a reasonable interpretation as one is a mass produced part that could have been fitted on the assemby line, the other is apparently a semi finished product that in effect has to be 'manufactured' by the installing gunsmith.
The Australian importer of SP01s is now trying to get CZ to fit the competition hammers at the factory, instead of sending them as a loose part. He also brings in Glocks and can fit the mag release, slide release and change the sights himself.