You're only partially right. 

To say B is a member of set A, is not to say that All B = A, (corrected to make your intended point)

However, 

When "some C" and "some D" and "some E" and "some F" (plus any other ethic group you care to list) all have members that are A, the act of singling out out B = A is indeed discriminatory.

Further, Trump initial statement did not assert "Some" (or some other indicator of a non-universal set) Mexicans are  drug dealers, but rather Mexicans are drug dealers. In colloquial English "all" is implied, but "some" must be specified. To assert that Trump did not include the "all" and therefore meant "some" is not colloquial English. And to point out that he later corrected himself by including the "some" merely highlights his racism for those who listen with psychologically tuned ears.