That applies only to people physically within the US. A non resident alien is someone on a work visa or student visa or other authorization. A resident alien is someone on a green card. Those rules don't apply to people in Canada who pay Canadian taxes(otherwise there would be double taxation).

E.g of companies handing Canadian compliance and billing their US clients. https://syndesus.com/why-u-s-companies-need-a-peo-to-recruit...