But can you legally cross the border at any of those places? Sure, you could “get to Canada” by swimming south across the St. Croix river in some places in Maine, but I’m sure you’d get in trouble with the Canadian authorities if they caught you. I’m pretty sure the expression is saying Detroit-Windsor is the only official border crossing where you travel south into Canada.
You can take a ferry from Ketchikan AK to Prince Rupert BC which crosses the border going south. I don’t know if that counts as an official border crossing or not. However, Prince Rupert is due south of the US and Ketchikan is due north of Canada.
Fair enough. You may have to add the qualification that it be a land border for the expression about Detroit to be true, or exclude Alaska.
Except it’s not a land border at Detroit. It’s a clearly a water border. Detroit even means “strait” in French, emphasizing the fact it’s a real water border. It took a long time before anyone built a bridge.