I think it's from what situation it is... when the Tongan Death Grip follows after a killing move, then it's a pin, when it comes sudden (like the RKO), it's a submission. There's a super-cool Tongan Death Grip hack for WWF No Mercy.
Jetlag, you must tell me this hack. And by the way, technically he used it as both. Kayfabe-wise it was said to be so powerful, that it could incapacitate even the toughest men. He'd slap it on and either they tapped on there were rendered unconscious and he took the pinfall. Either way, lights out.
Too lazy to search the link now. It's @ www.thenomercyzone.clicdev.com , in den Move-Hack's section, there's a turtorial section, where there's a stickied topic called "How to hack move turtorials", where theres a "Damage Mods" Turtorial where there is the Tongan Death Grip hack... uhm, was that now less work than just searching the link?