Had Meg Marquardt actually done some real research on the way paleontologists understand extinction events, she could have noted that the idea of a pocket of survivors is not a big deal and in no way completely overthrows the well-established evidence that the extinction was indeed massive.

"Miscalculation" is the wrong word. Geologists are nuanced (unlike this headline) in how they interpret C-13 vs. C-12, especially for early Earth history. The PNAS paper is not about the ratio being flawed, it's about another way to interpret the activity of the shallow ocean versus the deep, open ocean. The paper is simply another line of scientific discussion, itself part of the scientific method.