There are many different ways to show that a single photon is actually a superposition of both slits in the double slit experiment and this was a particular good one.
The author has done a really good job with his double-slit microscope with a HeNe laser and a CCD to image the diffraction pattern. He only missed a few details in his experiment, which also showed excellent single-slit as well as double slit-diffraction. It was very clever to simply lower the beam intensity in order to show single photon behavior and so this is an experiment that I could do with my microscope and laser as well.
In his explanation, he described a dipole source as spherical source in all directions, but of course a dipole is a planar and not a spherical source. This does not really change any of his conclusions.
He did not talk about the fact that the emitter and detector were in resonance for the lifetime of the emitter, which was about 1 ns or so for a 632.8 nm HeNe at 0.3 mW with a 1 GHz bandwidth. Each single photon has a 1/e coherence length of 300 mm and so the emitter and detector are close enough for quantum phase correlation. His diagrams incorrectly show very short photons while the actual HeNe photon is in fact much longer, especially as an amplitude, which is sqrt of the intensity length.
The single photon width corresponds to the 1.5 mm HeNe beam width and so a single photon always goes through both slits as long as the beam diameter covers both slits. Therefore, this is not a mystery at all and the true mystery is why does anything ever behave like a classical particle at all. The simple answer is that it is the decay of the quantum photon resonance that makes a photon classical. That is, it is the decay of this source-detector quantum resonance at the detector that makes the quantum photon a classical particle.
Finally, he mentions that the single slit diffraction also means that the single photon interferes with itself and this is true. He suggests that the single slit acts like a resonant chamber and this is exactly correct. In fact, there is a short and quite measurable delay in the photon transit through a slit because a photon lives longer in the slit.
All in all, a very nice demo!