They failed because the instructions are incorrect and cannot be parsed (assuming, as is reasonable, that they were given the instructions seen in the blog post). It's quite possible that the only student who succeeded did so because she misread the first sentence in just the right way. Note that the assignment has "the first of which is drawn from a point halfway between a point halfway between the center of the wall", while the student's version repeats "halfway between a point" twice, which makes the sentence parseable.

What really surprises me is that nobody reported the mistake to the professor.