It was just speculation, and looks to be incorrect, but not as crazy as you are making it out to be.
First, they easily could have hid the burner phone somewhere where the FBI wouldn’t find it. The FBI didn’t tear up the house, they did a standard search. I am sure they know a good place in their own house to hid things.
Second, Laundrie’s friends has said when he first went that he’s an experienced survivalist with extensive knowledge of that area and he could live there for weeks if he had to. There are many people who do that for fun, and are quite good at it. It’s not illogical that he lasted there for a few weeks.
Third, the fact that the parents did nothing when he came back without Gabby, and then took him to that area, and let him go out for a hike knowing that Gabby was missing, makes everything they do highly suspect.