There's a difference between ACK of the remote network stack (yeah, we got your packets, they're waiting in line) and ACK of the application (yeah, app X processed your requests composed of 1 or more packets)
Compare with the classic OS optimization for spinning rust hard drives - write system calls will return immediately, but actual write requests to the hardware will occur sometime later. It's assumed most of the time your computer doesn't lose power, but that does happen sometimes, hence journaling.
Compare with the classic OS optimization for spinning rust hard drives - write system calls will return immediately, but actual write requests to the hardware will occur sometime later. It's assumed most of the time your computer doesn't lose power, but that does happen sometimes, hence journaling.