The article is correct both that this is a bigger slice of the Federal prison population, and even then, not most of that population either.

But ask what percentage of African Americans are currently in prison, or have a record, due to drug charges, and the number shoots up. This is why one in 3 African American men can expect to go to jail in their lifetime, why one in four drug arrests is of a Black person: https://www.sentencingproject.org/publications/un-report-on-racial-disparities/

This is despite drug use being roughly equal across races. And while it's true that the majority of arrests today are not drug related, a lot of laws that increased police contact with Black people were based on the drug war trope that drug users and dealers were predominantly Black. 20-30 years of buy in from both Democrats and Republicans of this concept led to broad increases in arrests of African Americans, and not just for drug related crimes.