The S-box of a cipher acts as a substitution table. It forms one component of a symmetric cipher. This substitution, which is repeated many times in a typical cipher, acts to make the encryption less predictable and thus harder to reverse.
This e-mail alleges that the S-boxes used in the Russian cipher are not created with randomness in mind, but instead they were crafted to deliberately preserve a type of mathematical structure.
The author of the post suggests that this structure is very worrying, and it might leave the cipher open to an as-yet unknown attack.