They weren't meant to.
What they DO 'prove' is that - it's meant for all those who blare they have the true path while others are evil and going to hell.
The Mormons DO believe they have the "true path", as does every other denomination I've run across. They DO NOT believe the others are going to "hell".
To put it simply, they believe the others will go to a "paradise" where they can learn the true path after death - hence their "baptising the dead" practice.
They believe the only people who are really going to "hell" are those who have literally KNOWN God and rejected Him - like Lucifer did.
Some Christian theologies say when you're dead, that's it - which is kinda tough on those who never really had a chance to know and accept Christ.
Mormons do not.