The question is why these much higher CO2 levels didn't make the oceans a lot more hostile to life 500 million years ago than during the Great Extinction?
It's possible an expert on ancient climates and lifeforms has already figured this out, but it's a disconnection.
Here's something on marine extinction events.
The graph shown on the website is labeled and explained. "P-Tr" is the event discussed in the original article.