Probably referring to the key distribution process, where to enable iCloud Keychain you have to approve from another device. It's a sound design in theory - the keys are only stored locally, so even Apple can't access them - but I've personally experienced issues several times where the approval notification wouldn't show up on my other devices, or the UI was in an inconsistent state, etc.

It's worth noting that the iOS and macOS keychains are very different. In fact, if you check the design docs you'll find they share only four functions between them. Essentially, as far as I could tell, the iOS keychain participates in the sandboxing of apps, while the macOS one does not.