Wandering Spirits are not minions, but there are two ways in which they’re similar to minions:
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Casting Wandering Spirits counts as using a minion skill.
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Wandering Spirits’ damage is affected by “increased minion damage”, and other such minion damage modifiers.
I’ll amend the tooltip in game to make this clearer.
EDIT: as of April 2020 this information is outdated. Wandering Spirits no longer scale with any minion stats, and casting it does not count as using a minion skill.