While it's true that Canada is the US's largest supplier, the article does not state that this is just barely the case. The US's total imports of crude oil from each of Mexico and Saudi Arabia are often within 1% of the total imports from Canada. Hardly surprising if someone guesses either of those countries.
Approx % of Recent US Crude Oil Imports:
Canada - 16.9%
Saudi Arabia - 16.8%
Mexico - 16.5%
Venezuela - 14.8%