You’re thinking of another city, very likely Teotihuacán. Tenochtitlán was built from scratch by the aztecs. (Either that or please give sources for your statement).
Parent is probably thinking of Teotihuacán which is nearby. We do know who built it but there is relatively little know about them (compared to the Aztecs). The ruins of Teotihuacán were discovered by the Aztecs and they were mythologized, for instance even the name "Teotihuacán" is the Aztec (Nahuatl) name for the city meaning "the place where gods were created."
Maybe they are referring to the fact that the Mexica came from elsewhere and founded the city and there were already other people/cities in the area. It seems the Valley of Mexico has been inhabited since around 12,000BC, so not surprising there isn't history of the first people there.
