And Zoro was still standing after taking Luffy's pain, although Kuma said it was impossible for him to withstand it, wasn't he? There's a difference between double the pain and double the broken bones. The pain, Zoro can endure it, and since his bones are now healed and he can only feel the pain, he can still do stuff with that pain. The bones tho, it's essential for Zoro to do anything with bones, especially 60 brokens one.