Buttermilk is a marinade, not a brine. The calcium and acid in the buttermilk work to slowly break down the proteins in chicken to make it tender, but do so without the chicken absorbing a lot of water as would happen with a water-based brine. (Buttermilk also sticks to the surface of the chicken, helping breading stick and keep the natural moisture locked inside the chicken.) Differently from a brine, buttermilk is not adding salt or moisture to chicken - it is helping the chicken retain its natural moisture while also breaking down proteins to prevent toughness in the meat.