You’re probably mostly right, but you’re conflating Tennessee law, which is totally unaffected by this federal pardon, with federal law.
I’m reasonably sure Federal law doesn’t define felony possession of marijuana to include a specific quantity. (Neither does Tennessee law, if we’re being precise. See TCA 39-17-417)
But replace your references to quantity with manufacturing, delivering, selling, or possessing with intent to manufacture, sell or deliver and the whole thing is basically accurate.
There’s probably nobody in jail (although there are other benefits to a pardon than being let out of jail but for a misdemeanor like this they’re pretty minimal) and the FBI generally isn’t out looking for people who are just consuming.
Most 21 USC 844 violations are probably caught by the National Park Service and the GSA police(?) who enforce the law inside federal buildings, if I had to guess. Whether or not they arrest or cite you for it, I have no idea. That may have been another carry over from Tennessee law.