The Gemora establishes that there is a difference between the child of craftsman and a child of a robber in the case where there are witnesses that saw the father admit that he sold the property.
Tosfos asks on this that if witnesses saw an admission, even a craftsman and a sharecropper themselves would be believed!?
He therefore emends the text of the Gemora to read that the children claim that the original owner admitted to them that the land was sold to their father. In the case of the child of the sharecropper and craftsman, this claim is believed. In the case of the robber, however, it can be assumed that the owner admitted to the son because he was afraid of the father.
Rabbeinu Yonah defends the original text. He says that the case of the craftsman/sharecropper, and the case of the robber are not identical. The case of the craftsman/sharecropper is when they claim to have bought the property. In such a case, they are not believed. The case of the robber is when there are witnesses who saw the owner admit. This is teaching us a bigger novelty - that even when the owner admits in front of witnesses, which is a serious admission, it is still considered to be a false admission motivated by fear.