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

The only possible way would be to disable keychain syncing which would likely stop many Apple services from working. If you look at keychain it includes entries of may kinds. The entries for 1Password do not include your account password. They are encrypted. They contain your Secret Key which is known on any device you use. If it were not then you'd need to enter it on each device.

https://1passwordstatic.com/files/security/1password-white-paper.pdf

Our two-secret key derivation mixes your locally held
Secret Key with your account password so that data we store cannot
be used in cracking attempts.

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Encrypted copies of your Secret Key are stored in your device backups and keychains to provide data loss protection. If you have iCloud Drive enabled and lose your Mac, iPhone, or iPad, you can restore from a backup and unlock 1Password with just your Password.