It is a gery area interstating, but I did hear something that sounded a bit like:
If the load does not originate from the USA, Canadian and Mexican driver can do load within the US (i.e deliver)
Such as if a Canadian driver picks up a loaded trailer in Laredo, TX with an origin from MX (trailer still sealed) he can deliver the load in let's say Nashville, TN. This would not still be considered interstate. The same goes that if the same driver picks up a load in Memphis, TN and drops a the trailer in a transit yard in Laredo, TX (no delivery to a transit warehouse autorised, trailer must stay sealed when he is attached to that trailer)
From my understanding. (I hope I understood correctly)
But as far as an American driver driving a Canadian truck, I doubt that would be considered an Interstae, he may get stopped and verified while on route, but that would be all.