Except that all they and we know is there's a stone with their names on it. There's zero proof that they're under it. And even if there was, that just means they'd need to hold the funeral there.
It's no more a definite finality than the Doctor's gravestone in Revelation Of The Daleks was.
To be fair, it's been shown since the beginning of the new series (and it was hinted before that) that the Doctor has some kind of "time sense" that makes him perceive paradoxes, time shifts and other phenomenons. That's good enough for me: he saw the names on the stone, and he recognized that as a fixed point in time, and Bob's your uncle.