The first group was given the same instructions as the participants in Guilford’s experiment.
The second group was told that the solution required the lines to be drawn outside the imaginary box bordering the dot array.
It's an interesting experiment, but the author's conclusion cannot possibly follow from the results of it.
your conclusion: that the second experiment disproves the theory that thinking outside the box is useful in solving problems, is itself a fallacy.
Even though they weren’t instructed to restrain themselves from considering such a solution, they were unable to “see” the white space beyond the square’s boundaries.
Only 20 percent managed to break out of the illusory confinement and continue their lines in the white space surrounding the dots.
He challenged research subjects to connect all nine dots using just four straight lines without lifting their pencils from the page.