Lizardling got the gist of it. It is survivable if the spinal cord is uninjured. One way to look at an internal decapitation is as a really severe joint injury. A dislocated elbow or knee is not fatal. Same with an internal decapitaition, provided that vital nerves are not severed.
Events leading to internal decapitations are predicatbly flukey. Former NBA player Rudy Tomjanovich suffered an internal decapitation after being punched by an oponnent. The punch caught Tomjanovich more or less unaware … IIRC, he pretty much turned into tKermit Washington’s moving fist.