Dr. Tom McDonnell:

A currency has a price. All currencies have prices against each other. If there is a global recession, all prices might fall but they are all relative to each other. If your price falls by less than the price of the other currency, then your currency has effectively appreciated against it. Often during global recessions, the US dollar is considered a safe haven currency, so that can lead to appreciation. The key point here is that if Americans are buying fewer goods, there is less demand for the foreign currency. Therefore, the value of the dollar might increase and that might counter some of the terms of the trade benefit they would get from the tariffs in the first place. That is one of the assumptions. That has not happened so far and that is why there is a footnote in the opening statement to say that, in fact, the opposite happened.