The simple answer is that China does not want to use the Northwest Passage to ship to Canada. It wants to use the Northwest Passage as a shortcut between China and the Atlantic seaboard of the United States.
The world being a sphere, the optimal shipping route depends on where you start and where you want to finish. The northern sea route along the northern coast of Russia is optimal for shipments from China to Europe. The Northwest Passage, if it is free of ice—which is not yet the case but could be soon—would be optimal for shipments from China to the Atlantic seaboard of the United States. That's what they're looking at.
In an optimal scenario, they would then have a transshipment port, probably in Newfoundland. They could use ice-strengthened ships through the passage and then transfer to regular container ships at Newfoundland. The container ships would then go down to different ports in the United States.
I don't regard that possibility as necessarily threatening, because it's commercial and because we don't have a transshipment port in Newfoundland. Maybe we could partner with China in building one.
Let me be clear, too. I'm very worried about China in many respects, and I would not want to see them getting control over the port of Vancouver. However, if they wanted to take several billions of dollars from their polar silk road initiative and partner with the Government of Canada and the Government of Newfoundland, that is something I could imagine having a good discussion about.