Federal prosecution is rare. If they charge you with marijuana possession at the federal level, then there’s probably a list of a dozen other charges on top of it, and at least one of them is the serious thing they’re actually after you for. Feds don’t waste their time with this shit unless there’s a bigger reason.
So what Biden did has very little practical effect. A bunch of people got one charge among many taken off their record. That’s what he’s able to do with just a stroke of a pen. The rest is dependent on states, federal congress, or the vast federal bureaucracy.