I think he might mean that the "whiter skeletons" (the normal ones) are doing the wither affect. so the message is meant to go: When the whiter skeletons attack you they inflict wither too. so he is actually talking about the white skeletons with the bows not the black ones with the swords.
I think he might mean that the "whiter skeletons" (the normal ones) are doing the wither affect. so the message is meant to go: When the whiter skeletons attack you they inflict wither too. so he is actually talking about the white skeletons with the bows not the black ones with the swords.