I am from Canada. When I registered my KDP account, I filled in my SIN (Social Insurance Number). After registration it was indicated that "Tax Information is complete" and "Applicable withholding rate: 0%"
The question is: Do I still need to apply for EIN or TIN ???
Just bear in mind that this system couldn't work if the two tax authorities weren't exchanging information. You may be sure that the Canadian outfit will know all about your US earnings, so if they are taxable, be sure you report them!