At the first stages, all the participants in Guilford’s original study censored their own thinking by limiting the possible solutions to those within the imaginary square (even those who eventually solved the puzzle).
Although studying creativity is considered a legitimate scientific discipline nowadays, it is still a very young one. One of Guilford’s most famous studies was the nine-dot puzzle.
Guilford was one of the first academic researchers who dared to conduct a study of creativity.
For example, there have been some theories such as those of Schopenhauer (see his remarks about Genius) and Freud (see his remarks about Sublimation) that propose creativity is something more like a capacity provided by nature rather than one acquired or learned from the environment.
Rather than disproving the myth, in other words, the experiment might instead offer evidence that creativity is an ability that one is born with, or born lacking, hence why information from the environment didn't impact the results at all.