I'm not a Bungie official, but I'll answer this question to the best of my ability.
When people speak of Hive Wizards using witchcraft, they are talking about how they use the power of the Void and the Darkness to perform tasks and attack us. Now we refer to these as spells and Wizards pretty much always perform them by reciting incantations (such as the Deathsinger's song). In relative terms, this is almost exactly like the mythical witchcraft that people were accused of using in medieval times (back when people accused people they didn't like of being witches just so they could be burnt at the stake), except the Wizards are using the power of the Darkness in these rituals (called rituals because they follow the Darkness's philosophy religiously)
We call what the Wizards do witchcraft and use other words that we would associate with witches because that is the closest thing we can relate it to
I hope I answered your question properly. If I didn't, please tell me which grimoire u r referring to so I can hav a better understanding of the question