I actually think this is a plan to get rare earth resources.
I also think that it is an obviously extreme position that then will then obviously fail, but then the United States will fall back to "Canada and Greenland can only sell rare earth rights to the United States" to lock China out.
I heard that Trump is refashioning the CIA to focus on the West, which could be understood as including a focus Canada. I can sort of imagine the turbulence we may start to see in Canada's political realm as a result of such shift of the CIA's priorities and operations to Canada in the service of achieving Trump's goals:
> "His aide said Trump’s CIA will have a greater focus on the western hemisphere, targeting countries not traditionally considered adversaries of the US, according to the Journal."
There is a plan for that, take more land up north, e.g. Canada and Greenland.