You're confusing a subsidiary with a division or at least a wholly-owned subsidiary.
A subsidiary is a company whose majority stakeholder is another company, which is exactly what reddit is. If reddit weren't a distinct corporate entity, they'd be a division.
Now, since AP doesn't have 100%, reddit isn't a wholly-owned subsidiary, but a company doesn't have to be wholly-owned to be a subsidiary at all.
A subsidiary is a company whose majority stakeholder is another company, which is exactly what reddit is. If reddit weren't a distinct corporate entity, they'd be a division.
Now, since AP doesn't have 100%, reddit isn't a wholly-owned subsidiary, but a company doesn't have to be wholly-owned to be a subsidiary at all.