That is net exports between Canada and the US. Similar to how we were talking about Debt to GDP ratio, it doesn't paint the entire picture. The problem is a lot of manufacturing relies on parts imported from Canada and Mexico relying on those manufacturing bases to produce many finished products. Even cars that Ford and GM manufacture rely on parts and mechanics manufactured in both Canada and Mexico to manufacture and then are placed in the vehicle in the US and then exported. So no, it will hurt manufacturing and prevent the export of many products as the cost of manufacturing will increase in the US knocking down beyond the lateral trade you are referring to.
Also, you can't just look at the percentage of exports to figure out what helps and hurts. Just because it doesn't hurt as much, doesn't mean it won't. Specific industries get hurt tremendously because of rash decision making. Home builders are already seeing a relatively large drop in starts because of the tariffs on soft lumber. And the tariffs haven't really helped US producers. We have already seen German exports of softwood lumber to the US jump 1000%. Russia as well. The US didn't capture the lost domestic sales, it just went from Canada to other countries. So what help did this really do? The US exports large amounts of corn and soybeans to both Canada and Mexico. Canadian and Mexican cattle use this for feed, food industries etc. Sure, beef is then resold into the US, but Canadian beef also is exported to other countries such as Japan, S.Korea and China. Without the support of those purchases, the costs increase and North America as a unit will lose sales to Australia, NZ etc. The fact of the matter is a lot of the sales of these products are synergistic in capturing income from places outside of North America.
With NAFTA gone, these specific exports are in danger. When an entire industry is knocked, that ripples spending in other industries giving an overall domino effect. That is the tremendous danger. Angering the voters who actually rely on paycheques in these businesses are what Trump is in danger of losing. The fan club is not that big. But angering the farming and manufacturing voters (which allowed him to win key states over Clinton) is a major problem. Trump I believe is smarter than that.
You are correct in that Canada and Mexico will be disproportionate in getting hurt from the exit of NAFTA. But make no mistake, it will hurt many key industries in the US in which Trump needs to win over to hold onto his presidency.