it's a bullshit term because a managers contributions are also individual and the kind of contributions (implied to come from non-individual contributors) also come from individual contributors
ICs are given tasks for one person. Managers are given tasks to distribute to entire teams. By definition managers do not contribute individually because their output (from an org chart perspective) includes the output of everyone reporting to them.
regardless of who is on the hook for what, ones contributions are their own. a manager makes contributions by performing actions just like anybody else. this label for implying that a managers contributions includes that of their reports is bogus - if a manager does absolutely nothing at all we should consider the contributions of their reports? actual technical jargon adds precision to language "IC" takes it away, it's nothing more than corporate newspeak ala 1984