Canada and the US have some disagreements about the arctic, particularly about the Northwest Passage: https://brownpoliticalreview.org/2020/04/the-u-s-canada-nort...
That's got to be a factor here
> The US nationalists might be okay with it because of NAFTA
Why are nationalists ok with NAFTA, which is just like any other international fre trade agreement? Is it because Trump backed it - meaning, of course, that they aren't nationalists but Trumpists.
I agree with your economic nationalism comments. But Canada is in a special position here: the US Arctic is contiguous with Canada's much larger Arctic region and a US/Canada icebreaking partnership seems to make sense. The US nationalists might be okay with it because of NAFTA, generally close economic ties and the whole "fortress North America" thing. Canada has at least two shipyards capable of building icebreakers and the US has money.
Another example of this close, almost ambiguous economic relationship: the US Department of Defense is funding Canadian mining juniors. I have never heard of this happening before: https://www.defense.gov/News/Releases/Release/Article/377704...