According to Mexico. But, according to the US, he's an American citizen.
The USA does not recognize, nor concern itself, with the citizenship laws of other countries. You'd be advised to familiarize yourself with American citizenship law.
You are wildly confused. Whether or not a person holds Mexican citizenship does not depend on the US. Mexico gave Curiel his Mexican citizenship, and the US cannot take it away.
How did the Dallas Morning News know, with absolute certitude, yet WITHOUT access to Cruz’ immigration file, that Cruz was a NBC of Canada?