[_void_star_]

I'm not sure you right on this. The way I'm reading it they generate the keys by sending the same random signals through each set of glass. If they send N signals through the glass and N is >= the message length then it certainly is a one-time pad. The two keys end up being as long as (or likely longer than) the message, so you can't brute force it.

[duaneb]

You're assuming that the structure of the glass can't be 'compressed' with assumptions about the structure of the glass.... At least the key space could be reduced well below the naïve entropy estimate.