I think that's all bluster from the orange goon. I think that he just has a hate on for JT because he makes his wife wet. I highly doubt that they would want to open that can of worms, it wouldn't be pretty. It wouldn't just be Canada in the fight either. They would further isolate themselves too.
NATO may fall apart (Putin's goal all along), leaving Canada to fend for itself. He doesn't care about JT, he wants our arctic shipping lanes and our natural resources.
We are a commonwealth country at the least, those in the commonwealth would have out back. I actually doubt that even if NATO were dissolved, other countries would pitch in to defend our soil. They wouldn't just roll in here and take over, I would guarantee that.