I have little doubt that the OST was circumvented by definitional tricks. That is how the game of treaties and power is played. Something like rods from God or a conventional bomb orbiting platform wouldn't be considered a WMD but most certainly are the types of weapons common folk would think the OST forbids.

I suspect since the OST didn't ban orbital weapons capable of attacking the surface outright that some of the signing parties already had them and didn't want to give them up.