Why was I asked for only my master password (and not my secret key)?

Hello ThomasK! 👋

The account suggestion that you're seeing in the 1Password app on your Mac is being shown because the 1Password app has found a 1Password account in your iCloud Keychain. In order to make it easier to sign into your 1Password account on your Apple devices 1Password will store an encrypted copy of the equivalent of your Emergency Kit in the iCloud Keychain.

If you have ever used 1Password on another Apple device where you're using the same Apple ID then the email address associated with your account and your Secret Key were stored in the iCloud Keychain so that you can more easily add your 1Password account to your other Apple devices.

There's nothing to worry about here since this information is encrypted and can only be accessed by you. And to complete the sign in process you still need to provide your 1Password account password.

Let me know if that helps. :)