From the researcher: 'We estimated between $53 million and $99 million in annual health damages from associated air pollution — among the largest health-damage estimates produced for a single facility to date.' [0]
Still enormous, but there is a significant difference between being 'THE X of all time, and being 'among the X of all time'.
Regarding 'minor source', in this case the expected additional premature deaths in the research range from 3.4 to 6.5 annually. [1] At the same time, air pollution leads to about 100,000 deaths a year in the US. While tragic at an individual level, adding 0.006% to the annual premature death count is fairly minor, just as reducing the deaths to 999,994 would be considered a pretty minor fix to the issue.
[0]: https://hsph.harvard.edu/news/analyzing-air-pollution-health...
[1]: https://www.pecva.org/wp-content/uploads/Health-Impacts-Vant...
[2]: https://sustainability.stanford.edu/news/identifying-sources...