Alibek cannot keep a secret. Wasn't it the author Richard Preston who said that Alibek told him the secret of cheap and easy anthrax weaponization? Whatever impelled Alibek to blab even part of such a secret, and to an *author*? Would it be surprising if Alibek had given hints to everybody and his kid sister?
So Alibek gave hints to people, maybe enough people that somebody picked up on the clues and was able to figure out the missing parts.
So, even if the 2001 recipe *was* derived from a Soviet recipe, that doesn't mean that Russians were behind the letters.
Aside from the recipe, whoever mailed the letters had physical access to something very close to the Fort Detrick substrain of virulent Ames.