Holler cred is the local credibility or "street cred" one might enjoy in the mountains or hollows, particularly of Appalachia. This comes from knowing the terrain, being "bush wise," knowing how to hunt, fish and assault Federal officers or Revenuers. Writing songs, or singing them, about white supremacists and sovereign citizen domestic terrorists also helps.
He fights mountain, talks mountain and has a still in his back yard. He sure has holler cred.