Who exactly didn't know this? It was common knowledge that Mazda was 4 cylinder building engines for Ford everywhere in Michigan. Heck they had a plant in Michigan that was a Mazda plant that built only Ford engines for a while. The Numi plant in California was a Toyota/GM(Pontiac) plant till Pontiac ceased to be and that was no secret. I don't recall any of that being "hidden" or misrepresented in any way. It's been out there for a while. If you as a consumer didn't pick up on it or missed it, then that's your deal. You also have it pretty wrong in regards to who "built" the cars. Components of the cars were done by other manufacturers, but final assembly was done by the actual maker of the cars unless you are talking about europe. In the state, even in the Numi plant, Toyota workers built the Toyota and Pontiac workers built the Pontiac.
I think, considering they have been making cars for a LONG time, automakers have all the reason in the world to blow off google. It would be like patient telling a Doctor what technique he should use for an operation.