US Withholding Tax

No you are not. Withholding tax is based on where the company is domiciled, not what currency the dividend is paid out in.

Because WCN is domiciled in Canada, the dividend is not subject to withholding tax despite it paying in USD and 85%+ of the business being in the US.

I'll give you a situation where this would have changed in the past, and that would be when Encana moved its headquarters to the US from Canada in 2019. Now, when you hold Ovintiv, despite it being a Canadian listed company and ticker, you pay US withholding tax because it is US domiciled.