The evidence is that everyone seems to be referring to 'the Hound' as a sort of persona or title (which is a bit weird), so when we are told that the Hound is dead but then another Hound (the guy Brienne kills later on) shows up, that throws into doubt that the septon was talking about Sandor. You then have the gravedigger making friends with Dog (Dog = Hound, not one of GRRM's subtler moments) and him being able to tend Sandor's horse, despite Sandor repeatedly saying throughout previous books that his horse won't accept anyone else than him touching him, and I think you have a reasonable case.