It's not common sense. Canada could officially look at me as a Finnish Citizen because as is the case with many Canadian citizens who are unable to apply for TRV if they lose their Canadian passport because their other previous nationality stops being relevant once they have obtained Canadian Citizenship. Same logic could be applied to ETA visa holders that once you obtain an ETA, your other past nationalities stop having any relevance. Passport is just a travel document and not so much a proof of travel privileges in such cases.
Also I wonder how the members of this community are so absolutely sure about everything? Has anyone ever attempted travel on ETA + secondary passport from non-visa exempt country? My understanding is that it could be possible depending on the knowledge of flight boarding staff and how they feel on the day. Nothing is as black and white as this forum seems to suggest.