Apple had received 10 out of its 15 requested exemptions for components like partial circuit boards. While Apple has a network of US suppliers for its products, many of the parts for computers (and those of rivals) are still made in China -- the company wouldn't have seen much benefit from US assembly if it had to pay a premium for some of the Mac Pro's key ingredients.
CEO Tim Cook (who hinted at this possibility in July) touted this as part of Apple's existing commitment to American jobs, including its recent investment into Corning. However, it's not necessarily the coup it sounds like at first blush. Apple can produce the Mac Pro stateside due to both its low volume (few people will buy a $6,000 tower for home use) and the high levels of automation at the Austin plant. This won't lead to an abundance of new jobs, and it may still be more practical to make high-volume products like iPhones and MacBooks in China even if future tariffs cut into Apple's profit margins.