I think the article is not precise with their wording, but the 97% is actually recall (i.e. detects 97% of the positives).
1 - 200/6158 = 97% is indeed a pretty suspicious coincidence though. I would assume/hope that they've shuffled a big dataset of recorded heart events (like the image in the TC article) and that the 200 people diagnosed with paroxysmal atrial fibrillation only rarely experience AF, so the number of true positives is probably far smaller than 1% of the dataset.