I think you're the one propagating a myth. Smith swept the states of the Old Confederacy, where Catholics were about as plentiful as hen's teeth. Hoover kicked Smith's crooked butt in the rest of the nation (including the heavily catholic Great Lakes states and Northeast.
NB. Of course, the Klan intensely disliked Smith, but by the 20s, they were strongest in the midwest, not the South. And there were a few southern states that Smith lost in 1928, but in general, your observation is right.