Sephardim in America were descendants of the Jews that were thrown out of Spain. 

The original Sephardim in America came from Amsterdam. Sephardim means Spanish Jews. These were what would be called today Ashkenazim-- European Jews not from N Africa 

That is not the same as Sephardim today that got mixed with Jews from North Africa. 

The Jews from N. Africa were intermarried with Arabs from the beginning of the Arab conquest of the Middle East. That is the reason Jews like Maimonides from those areas used the term ס''ט ספרדי טהור To show that they could trace their lineage to Jews. [Or to be a little more exact the term was used to show they use trace their male lineage to Jews. The distinction came about because in the beginning of the Arab Conquest Jewish women were taken as wives by Arab men.]

This is also the reason you find the letters  ס''ט after the signature of people like Bava Sali .

i would not make a big deal out if this if not for the fact that Sephardim make a very big deal out of their claim to be the only true Jews.  This seems to be a case of projection.