Just got a reply from DJI support. Although they didn't exactly contradict their first answer to me, they did clarify their position. Seems warranty will be honoured both sides of the border.
From DJI support:
With your concern, we only have one repair center and that is in Carson California, what we have there in Canada is an authorize retailer store.
If you have issues with your unit that needs to be repaired or check we can process it here since our shipping label covers North America and Canada. This will be no charge if the unit is still covered by the standard warranty when you purchase the unit.