I don't believe that it is important whether Canadian oil is being shipped to China or not. For every gallon of Canadian oil that is shipped to the United States, the U.S. will buy one less gallon from Saudi Arabia. Instead, that gallon from Saudi Arabia will be shipped to China.
On the other hand, let's assume that Canada eventually builds a pipeline to its West coast and ships oil to China. For every gallon that Canada ships to China, the U.S. will simply buy an extra gallon from Saudi Arabia and China will buy one less. The point is that it makes no difference whether Canada ships oil to China or not.
Furthermore, Canada is merely one player among many in the energy business. As a consumer, China has the option of choosing the source of energy that it prefers. For example, Australia exports 40 million tonnes of coal to China (see Ship Chartering: Australia plans to boost coal exports to China). China can always import more Australian coal and convert it into oil in one of China's coal-to-liquid (i.e. CTL) plants.
From a strictly economic/business viewpoint this is correct. However, from a strategic viewpoint the picture is fuzzy because supplies can be constrained whether there is an abundance of supplies or not. Saudi Arabia is a client state of the U.S. while Venezuela is not even though it is geographically in the Western hemisphere. China knows that in any conflict involving the U.S. that America will use its influence to stop or at least constrain the flow of oil into China. So, it is definitely not as simple as you are making it sound.
This logic may work for a country like the U.S. or its allies under its protective military umbrella, but for everybody else, they need to secure their supplies either militarily, through exclusive supply contracts or both. Do you really think China would leave their energy security at the whim of the U.S. by becoming overly reliant on distant oil suppliers who are either overtly or covertly American allies or client states? Most countries follow this logic and is the reason why we hear so much talk of militarily securing supply routes. Take the example of India. It is clearly building up its navy to attain the capability to stop Chinese oil supplies in case India instigates a war with China.
I believe China must be factoring these variables into their calculations and including additional costs for extra military forces required for safeguarding the supply routes. I would assume some calculation would be done to ensure minimal supplies could be guaranteed overland into China from Central Asia, Russia and Southeast Asia. Any supplies above this threshold could be sourced from overseas where their supply disruption would not be crippling.