I'm trying to work out a slightly tricky one here - can anyone advise?

This fact, below, is on the Museum of Wales website:

“These forests are thought to have been responsible for extracting nearly a hundred thousand-million tonnes (100 gigatonnes) of carbon from the atmosphere every year, and would have had a profound influence on the composition of the atmosphere during Carboniferous times.”

[https://museum.wales/articles/1243/How-coal-cooled-the-climate-300-million-years-ago/]

How accurate do you think this is?

Would love some input on this and if anyone knows whether there has been much work done on calculating the amount of Carbon locked away as coal, in the carboniferous. (whether thats each year, or across the whole period).

Many thanks in advance.

Grace