You save a password to the keychain in safari, and then you go to the “passwords” system preferences panel and you see those passwords. Or the other way around—creating a password from system preferences makes it accessible in browser. It appears on the surface to just be a password app, not a safari bucket.
It also works this way if you use the iCloud passwords app from windows. All of your saved passwords show up there as well, despite the absence of safari.
I’m pretty sure the new Chrome extension also shares your existing passwords.
I think it’s a reasonable assumption that when an app claims to use the keychain that you will have access to all your previously saved passwords.