That is why: "The value of the coin is given for business accepting WDC as payment." Right now USD and BTC are more valuable because that merchant accept them as pay method.
If he accept WDC too, then the value of WDC is bigger.
All of this without knowing it's real price.
The world buys products all the time from merchants that only accept their local currency, this is why currency exchangers exist, crypto is no different. It is totally naive to assume a merchant needs to accept a particular currency. If I jump in a local taxi and try to pay in USD or BTC or even WDC he will kick me out, I have to use an exchange to obtain local currency first, and that in no way undervalues the currencies I do hold, with the exception of the small conversion fee.