That's exactly what I was thinking. But is it officially 'sterile'?
No, because if one is denied entry to the USA by CBP, then one is ejected back into Canada, and will be required to immediately report to Canada Customs.
A passenger will not be allowed to board the aircraft overseas, destined to Canada, if one is known to be inadmissable to Canada for any reason. Even if they have an onward connection to a country where they are admissable.