I was confused by this, as well. Given the scale of geologic time, I'd assume they meant 300 million. But a single "M" often means thousand, especially if it's lowercase. So it's confusingly worded.
Most often in financial accounting, though conventionally, in other domains such as academia. Depending on the news outlet, some magazines and papers will also use "M" for thousand and "MM" for million, though that practice is probably dying out in the popular press. And of course, the "M" in "CPM" stands for thousand.
"M" was originally an abbreviation for "mille," which is French for "thousand" (by way of Latin origins). Traditionally speaking, one "M" means thousand, and "MM" means thousand-thousand, or million.
