ولا يشترى بها رقبة تعتق
Translation: Zakat money is not used directly to buy an enslaved person and set that person free.
The book distinguishes direct purchase-and-manumission from financial assistance to a qualifying mukātab.
The two are different legal transactions under the Hanafi transfer-of-ownership analysis. This is a historical legal discussion, not a description of a modern lawful commercial practice.