There is a difference between the two.
Sanji attacked Oven from the rear. When he kicked Oven he was outside of Oven's body and out of his line of sight. Sanji kicked across of his body and him near the collarbone. Look at Oven's left leg and compared to the location of Sanji's body.
Zoro attacked Apoo from the front and with a single swing he cut through Apoo tonfa, cut Apoo and didn't cut the anitdote. Instead he knocked the antidote out of his clothes. All thanks to us ability to cut nothing. Zoro attack was a frontal attack and he cut Apoo on the white part of his clothes that goes down the center of his body.