WWW
I do not doubt the letter you have received from a FSDO however I can find no basis for this opinion in US law.
Then I suggest you look at FAR61.3 a (1)
However; when the aircraft is operated within a foreign country, a current pilots licence issued by the country in which the aircraft is opertated may be used
So it is perfectly legal under US Law. FAR 61.75 which you quoted has nothing to do with it.
I would have expected anyone with any knowledge of operating N Reg outside the US would be familiar with this. The FSDO ruling was that "a current pilots licence valid in the foreign state" was covered under the existing law, even if it was not in accordance with ICAO Annex 1 i.e. IMC Ratings and NPPLs.