Hmm, I first thought he generally has "show my online status" disabled, but ctrl+f reveals: He does show up in the small text. Weird. Good find.
Because he wasn't marked as being a staff member (because in XenForo being marked as an admin, moderator, and Uber Employee doesn't mean they're listed as "staff").
I ran through all forum permissions, cleaned up administrators, etcetera. Everyone who is part of Uber should be in the list now.