There are several ways of thinking of this scheme, instead of thinking of it like a tax try thinking of it as just another commodity to be traded, hopefully that way it becomes obvious how it is not a tax.
The WCB simply exists to supply credits if the entire world has a deficit, which may happen when the scheme is first introduced but hopefully will not happen once the scheme is up and running. Once the WCB is taken out of the equation how can this possibly be a tax? It's just another commodity (in fact you could view it as a way to stimulate trade)