It's the difference between specific and general monitoring.

The BBC can absolutely take an individual login to iPlayer that they think is suspicious, and have TV licensing determine if that address has a license.

But it is objectively true that you can type in any old nonsense into a BBC account registration and it does not validate it against the License Fee database - that's because moving to a general system of checking is not what that database would be collected for, and would probably not be found to be a proportionate or non-general monitoring and therefore cannot use that exemption.