You either read or remembered wrong, or that source was referring to something like an explosives-grade uranium dart being dropped through one or more explosive

You either read or remembered wrong, or that source was referring to something like an explosives-grade uranium dart being dropped through one or more explosives-grade uranium donuts and from a great height. That method would not work for plutonium, which -- because of its greater propensity to prematurely generate neutrons -- requires the more-rapid mass-assembly method of an implosion.