On the most charitable level, the "huge math error" occurred in the calculation of the FDA's limits on exposure, so if the group feels that the FDA set its limits too high and the amount of chemicals being leached is worrying, then the math error really doesn't affect their conclusion at all.

But digging into the study, it becomes clear that the authors are really reaching in their methodology--subjecting utensils to rather implausible scenarios (when was the last time you microwaved your sushi tray?)--to try to get some samples that would exceed the limit. The hyperbolic media coverage of "ditch all your black plastics" isn't really sustained when the research finds that just a few specimens, when subjected to unusually high heat stresses, managed to just barely cross the incorrectly-calculated FDA threshold (itself likely set at a factor of a tenth to a hundredth of the lowest threshold observed to cause issues in previous studies!). That the authors aren't willing to disavow that media coverage is more telling then their unwillingness to adjust their conclusions in response to their math error.