Actually, I think that's pretty good understanding of the concept by TSA standards.
I agree; "Canadian thing" is not too bad of an explanation coming from a TSO, LTSO, or STSO. NEXUS is technically a program designed for frequent travel between Canada and the USA, by citizens/LPRs of either country. The airport iris scanning kiosks are located ONLY in Canada.
Remember, also, that the NEXUS card does not include the name or logo of either of the federal issuing agencies (CBSA or CBP). All it says is "NEXUS." I'm frankly rather surprised that TSA has decided to accept the NEXUS card (despite its being a WHTI-compliant document) because it does not show the name of the agency/agencies that issued it.
(Try opening an account at a small local bank in southern Alabama with the NEXUS card as a form of ID.....if it is recognized and accepted as valid ID, I'll give you $20.)