"If not X" is rhetorical, elliptical and parenthetical and has the meaning of "if not, indeed," or "and quite possibly."
“I have enormous faith and confidence in Merritt Paulson, who’s built from scratch one of the great sports teams, in any sport, in our country, if not throughout North America,”
can be restructured as
“I have enormous faith and confidence in Merritt Paulson, who’s built from scratch one of the great sports teams, in any sport, in our country - and I wonder if it is not true that [he has built from scratch one of the great sports teams] throughout North America.”
which, more lucidly, is
“I have enormous faith and confidence in Merritt Paulson, who’s built from scratch one of the great sports teams, in any sport, in our country and quite possibly throughout North America.”