That's the other thing that confuses me - If you want to discourage someone from investigating the Church of Jesus Christ of Latter-Day-Saints why would you tell them if they pray about the Book of Mormon they will receive a confirmation of it's truthfulness. What else can "That's how they get you!" mean? And since you are praying about it, who's going to provide the confirmation but God? One human cannot provide a life-changing confirmation of the spirit. That only comes from God and through his spirit.
The Book of Mormon offers the same message this sign does:
What does this sign say to you?"And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost." -Moroni 10:4