I instantly distrusted the assertion (it falls in the general class of "other people are idiots"-theories, which is always more popular among the Internet geek crowd than they should be), and went to the linked article:

The Piagetians used what they called a clinical interview to determine which reasoning schemes a child had mastered. They posed questions of the children and then asked about how they arrived at their answers. As mentioned above, the elementary reasoning schemes (classification, etc) were what were being used.

Because each clinical interview took two or three hours, it was only possible to get data for a small number of children. Some psychologists decided to try to create a simple pencil and paper version which could then be administered to many children and thereby obtain data about broad classes of children.

This already suggests that the data should be noisy. I can think of at least two problems:

1. The test only determines, at best, what methods the individual used to solve this particular problem - and, at worst, determines what methods the individual claims to have used to solve the problem.

2. The accuracy of the test may be greatly reduced by the paper-and-pencil administration thereof. Any confusion which occurs by either the evaluators or takers will obscure the data.