Layer thickness aside, the main reason for the “invisibility” undoubtedly is the extremely small grain size. Let's not forget, if you develop this kind of emulsion with a physical or colloidal developer you'll get beautiful yellow filters that absorb light from UV - blue. Looking through such a layer at a point light source, you will notice no haze.
There's practically no absorption at any visible wavelength. For example even at the 633nm recording wavelength for a red sensitive emulsion overall absorption is extremely low – though there must be high absorption on each light hit (spectrally sensitized) AgX crystal.
By the way, the energy levels we're dealing with, are pretty weak most of the time. A great many people are doing their exposures with lasers of a few Milliwatts of power only.