This is entirely frame rate sync (strobe effect). There is no motion blur because some video cameras have extremely fast shutter speeds, for example 1/250,000 of a second. Yes, one quarter millionth of a second. So even if the blades are going 1000 miles an hour, the blades have moved less than 0.1 inch during the shutter time.
You can clearly see the blades are not perfectly stationary. The sync drifts slightly as the blades speed up and slow down. So it's not an impossible coincidence that they are perfectly in synch, it is simply in the range that the blades look fairly stationary.