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

Apologies for the late reply. I have a bad habit of disappearing from threads.

Thank you lmilsfsd for your explanation. I do understand that iCloud Keychain is supposed to be end-to-end encrypted, but I'm a little paranoid, so I still don't want to be giving companies data that isn't necessary to operate their services. I'm no security expert, but the saying "no computer system is 100% secure" always goes through my head in situations like these.

I could send Apple feedback, but I get the feeling that would be like screaming into a void.

1P_Tommy It seems like you're on a different page than lmilsfsd and I. I don't want to disable iCloud Keychain entirely; I just want to prevent it from sweeping up my 1Password info. But based on what lmilsfsd is saying, that doesn't seem to be possible at the moment.