Bizarre password request while modifying keychain
This request comes because you have enabled Keychain for the first time with that account on this device. In order to get a copy of the current Keychain stored in the cloud, you need to authenticate yourself as the "owner" of that Keychain.

So in order to unlock the contents of your old Keychain, you need to verify that you're the same person that once uploaded that Keychain, by entering the unlock password you used on that old computer.

There's really nothing bizarre about it, nor it is any sign of a problem. It doesn't mean that your old computer is linked to your Apple-account as a device anymore - it simply means that the old Keychain data is encrypted, and you'll need the unlock password you used at the time, to get to the data.