As I read the SL it states, and I am sure Cessna will not mind a quote here,
"The Carry-Thru spar inspection presented in this service letter applies to airplanes listed in the Effectivity Section. The compliance time is for unmodified airplanes, and represents the maximum allowable time. Airplanes that have been modified to alter the airplane design, gross weight, or airplane performance, may need to inspect sooner. Examples of common supplemental type certificates (STC’s) include vortex generators, wing cuffs, STOL kits, wing tips, and add-on wing fuel tanks. Contact the STC holder for revised inspection intervals.
MANDATORY. This service document must be accomplished within 10 flight hours from date of receipt as follows...>"
So I would say it is not strictly true that it "does not apply to modified aircraft." If you have any modifications it could well be that the ten hours to compliance will be less. And I'll bet that most of the STC holders have yet to do an analysis on the the stress loadings for their mod. If I had tip tanks on a high time C210 that has not been eddy current tested I would put it on the ground now and get it done.
In some countries this SB has been done at (I think) every 2,000 hours as a mandatory procedure for years.