How did 1Password automatically know my account on a new Mac?

Swerve

Thanks for the reply. Just to clarify, iCloud Keychain isn't "sweeping up [your] 1Password info", instead 1Password is deliberately storing encrypted account information in the iCloud Keychain as a convenience and data access preservation feature. The information contains an equivalent to the contents of your Emergency Kit: your account password, your email address, and your Secret Key. It does not contain your account password.

It's also important to remember that the Secret Key alone is not enough to access your 1Password account. Someone would need your 1Password account password in addition to the Secret Key to log into your 1Password account and decrypt the data within it.

Apple's iCloud Keychain is very secure. The data stored in the iCloud Keychain is secured using end-to-end encryption which means that no one, including Apple themselves, can see what's being stored there.

If you wish, you can also enable two-factor authentication for your account to add additional security: Turn on two-factor authentication for your 1Password account

-Dave