It is true that in the traditional double-slit experiment the detectors were placed at the slits in order to observe the particle like behaviour. The writer is referring to the set up for Wheeler's delayed choice experiment, when the detectors are actually placed behind the screen and the decision of whether you would raise the screen (and hence detect which slit the photon passed through) is made after the photons have passed through the slits. Do you think we should make it clearer that this is different from the traditional set up? Let us know and we'll have another look at the article.