This is a purely theoretical exercise, from what I could understand:

[They] considered what would happen if a single photon passed through an optical shutter—essentially a very fast mirror that can be switched on and off to block part of a pulse of light. If the shutter was fast enough, it could intercept the photon mid-pulse, snipping off part of this extended wave.

They then did a bunch of calculations to simulate what would happen, but I'm wondering if such a shutter wouldn't have to travel at the speed of light to catch the photon, and if this doesn't make the experiment meaningless...