Wrong. A polygraph conducted in a stressful situation can absolutely be inaccurate, so there's NO reason in the world to think that the Ramseys not agreeing to one given by the police is a sign that they are guilty of anything.
The first thing the Ramseys did was to "lawyer up".That and other things strongly suggest that they know what happened..even if neither of them actually did it.
The story that this clown in Thailand is telling is already collapsing.When the collapse is complete the case will be back at square one.