It's pretty misleading to look only at federal inmates as they're a relatively small percentage of the overall prison population. There are currently 189,520 total federal inmates (https://www.bop.gov/about/statistics/population_statistics.j...) out of the total 2,173,800 incarcerated individuals (as per original article) in the US. Probably better to look for a figure that includes state prisons and local jails as well.
My post mentions both federal (about 11,500) and federal + state (about 20,000-40,000). The reason for mentioning federal is because BJS has data for marijuana versus other drug offenses for the federal system.
Note that while only a small proportion of the overall prison population is in federal prison, a much larger proportion of drug convicts are in federal prison.
"There's about 95,000 people in federal prison with a drug offense as their most serious charge. About 11,500 of those charges involved primarily marijuana. (The overwhelming majority of that is for trafficking, not possession.)"
So no, your statement is not correct. If we take 'overwhelming majority' of 11,500 as 60%, that would leave ~5000 prisoners whose 'most serious charge' is possession of marijuana. Or about 2.5% of the federal inmate population (all this assuming that the figures quoted are correct, I didn't source them myself).
Edit: here's a handy infographic: https://www.prisonpolicy.org/reports/pie2016.html