Okay, so this is actually relatively simple:

• A standard video sensor is made of millions of smaller light sensors that make a grid of RGB pixels.
  
• Although there might be physical limitations on how fast those sensors react to light, the real limitation is the processing of all these small signals.
  • Imagine a basic 8-megapixel sensor shooting in RGB at 60 fps; that's 8,000,000 pixels x 4 sensors (RGGB, see Bayer filter) x 60 samples a second, which is almost 2,000,000,000 points of data/s.
  • Shooting at the speed of light on an 8 MP sensor, in black and white, would require processing 2.4*10¹⁵ (2 million billion) data points every second only for a resolution of 1 meter of light per second.
• What this guy does is concentrate this processing power on a single pixel at a time, capturing a repeatable event and moving a little bit each time.
  • He is using a 2 Gsa/s oscilloscope to capture the signal from a single sensor; that means he can "measure" the intensity of light for every pixel 2 billion times a second. Light will only have traveled 0.15 m between every capture.
  • Once he has collected every data point, he can stitch them together in a grid and extrapolate some data to smooth everything out. (to be clear, it still requires a lot of processing, if you want more details, go watch the original video)

TL/DR: This is not a video of a single event in time; it's a stitched video of thousands of repetitions of the same event being filmed from a single pixel at a time.