That's not what "monopoly" means. Monopoly power isn't just a fancier word for market power. Monopoly power is power derived from the fact that nobody else is selling what you're selling.
Note that in this example, the Saudis can't raise the price of oil above $50 / barrel. Setting a price of $60 would send their sales to 0 rather than to whatever the level of demand is at $60. They can't do it because they don't have monopoly power.
(If you expand the example a little, saying that world oil demand exceeds the amount Canada can produce, then you can claim that Saudi Arabia has monopoly power by virtue of being able to sell oil that Canada can't sell. But in the model where Canada can produce any amount of oil at $50 / barrel and Arabia can produce any amount at $20 / barrel, there is no monopoly power anywhere.)
