You can create a local account during setup, or after setup
I believe you are mistaken in that one can create a local account during the 8.1 upgrade setup.
At least, when I upgraded, the process is that it is mandatory to create an online account. It caused me some minor hassles as I was in a bit of a rush at the time and didn't expect this curveball.
But, you are right in that you can change this after setup. So, it is just poor setup design rather than a cunning conspiracy to get people to use online accounts.
Disconnect from the Internet at this point of the upgrade then