The person is saying that the idea is not their own, but that it came from Erik Johansson. Therefore they want to acknowledge (express, confirm or admit) this fact, and say that "all credit goes to Erik Johansson." Credit can mean "public acknowledgement or praise," which is what is going on here. "All credit goes to him" means I am not taking credit for this idea, but saying that the credit/praise for the idea goes or belongs to Erik Johansson.