Primaries in Mississippi are open, and it looks like Presley's support here is coming almost exclusively from self-ID independent/other voters (Democrat was not an option), non-white voters (18% of this sample - huge for a deep south Republican primary), and from Bennie Thompson's district. Given that there is recent history of high non-Republican participation in Mississippi GOP primaries and there being no seriously contested statewide Dem primaries this cycle, I'd say this is generally pretty believable given the players involved.
It's certainly an impressive number and implies that Presley is building up some serious strength but it's probably exaggerating his crossover appeal right now.