The behind the wall channels are to far below the TV area to help. There are no other cross members in that area. If your really into that area for the mount you could use rivnuts for the mount....even mount a thicker doubler to spread the forces, then still use rivnuts for the mount.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."