Where is this NPPL website?
The authority issuing the NPPL has the power to restrict it as they wish.
They could limit it to G-regs. If they haven't then it is valid for any reg unless thus banned by the State of Registry.
They could require the pilot to wear pink underpants while flying, etc.
But they have no authority for interpreting the FAA regs. I don't see anything in 61.3 which states the pilot papers have to be ICAO compliant. Also the FAA reply re the IMCR (that URL above is from my
website) was a response to a clearly stated question and it was affirmative.
Obviously it's possible to get a different reply from some other FAA employee, but this happens with the CAA too, and much more so nowadays than a few years ago. I know somebody who sold a G-reg plane when the CAA told him he cannot fly it on an FAA PPL (which at the time was bollox) and we was not happy when I told him about the ANO article permitting it.