It's a bit absurd to have an article about this and not include any screenshots of the actual white blocks used by the lightgun to detect targets. This is a better explanation with visuals; it actually used two screens to do the detection:
It's also well explained (waaay more technical and in depth) by the "Retro Game Mechanics Explained" channel on YouTube: https://youtu.be/2Dw7NFm1ZfY?t=975
Simply a great example of using engineering and inventiveness to solve the problem they had. I remember noticing the screen having a flash, but because it coincided with the 'bang!' noise it felt natural.
so could it easily be tricked by putting a finger over it and quickly removing it in front of a light? or it's too small of a window to trick? a strobe light could fool it?
There's a timing element to it too. Because CRTs display pixels from top to bottom the timing gives information about the exact location of the hit too, not just a boolean hit/miss.
Have to detect a frame of black before detecting a flash of white. And the white has to be detected on the right raster lines. It's not trivial to fake.