The concept being used in this example is the same as that which is used for perforated vehicle window wraps, frequently used for advertising on buses and business vehicles.
According to this web site, Streamline Print and Design, perforated vinyl is the key:
Perforated vinyl is exactly what the name says, a sheet of vinyl
material with a pattern of tiny holes cut through it. There are so
many of these holes that a significant amount of the material is
removed. The amount of vinyl removed usually ranges from 30 to 50%.
For example, 60/40 vinyl, is 60% vinyl and 40% holes.
Light-colored perforated vinyl signs are opaque when seen from outside
a window because our eyes naturally focus on a bright, well-lit
surface rather than on the holes and the relative darkness of whatever
is behind the surface. The inner side of perforated vinyl, however,
is dark colored. Here the eye naturally focuses through the dark
vinyl to the light and motion outside the window. Because of this,
people inside a store, home or automobile see through the back of the
sign to the world outside.
I learned a bit of new info from this page. The idea of dark inside and light outside makes sense, but is not necessarily intuitive, especially in the case of masks and eye holes.
One aspect not covered in vehicle wrapping is that the closer one's eyes are to the holes, the smaller they can be. This can be demonstrated with a piece of black paper and pinholes placed in various patterns and sizes. Peer through the holes and you can see (or not see) that even tiny holes will provide good viewing.
It's not unusual to see costumes, specifically sport mascots, with large black gauze panels through which the human bean within will view the surroundings. The gauze is mostly holes, a very small percentage of area which is not holes, but the darkness inside and especially the greater distance from the panel to the human's eyes provides "invisibility" to the crowd. This does not apply in the case of a close-fit mask.