Might sound like a technicality, but: iCloud Keychain can store the passphrase but can't store the key itself. You still need an encrypted private key on-disk to use this: https://apple.stackexchange.com/a/250572 .
1Password saves the key itself in the encrypted vault and implements an SSH agent that can then interact with OpenSSH etc. and provide key operations, like how a physical dongle would function.
Ah, that is a very important distinction. Thanks for clarifying! For my purposes, the passphrase storage works fine, but I can see how the vault could be a useful feature.
