In other words, the “trick” was revealed in advance.
Management consultants in the 1970s and 1980s even used this puzzle when making sales pitches to prospective clients.
The symmetry, the beautiful simplicity of the solution, and the fact that 80 percent of the participants were effectively blinded by the boundaries of the square led Guilford and the readers of his books to leap to the sweeping conclusion that creativity requires you to go outside the box.
The idea went viral (via 1970s-era media and word of mouth, of course).
Because the solution is, in hindsight, deceptively simple, clients tended to admit they should have thought of it themselves.
Because they hadn’t, they were obviously not as creative or smart as they had previously thought, and needed to call in creative experts. The nine-dot puzzle and the phrase “thinking outside the box” became metaphors for creativity and spread like wildfire in marketing, management, psychology, the creative arts, engineering, and personal improvement circles.
It's an interesting experiment, but the author's conclusion cannot possibly follow from the results of it.