You already have to disconnect form the network to be able to use the "offline account" option in the installer (last I checked, which was like a couple of months ago).
So this has been going on for a while.
Edit (merge the other comment I deleted):
To clarify, this was a Home Edition. I understand that the Pro edition doesn't behave this way.
Neither you or your parent is incorrect; this behavior is variable. I setup 8 new win10 boxes last week, 2 of them (HP, the others were Dell) required pulling a lan cable to get to the local account setup. I didn't have time to check details to see what was different (I'm not really IT, but small company/many hats) but the local account option didn't appear until the internet was disconnected.
Windows 10 Pro, I've installed it about 40 times or more. Domain Join is the option to create a local account. It works, I do it every install, while connected to network.
I last installed Win10 a few months ago and it was not, until I unplugged the Ethernet cable and went back a couple steps to get it to re-assess things.
Installed Windows 10 Home a few weeks, poked around for 5 minutes searching for an offline option until I gave up. I know it used to be there but they have removed it.
I think it's possible in professional edition but you have to be offline on home edition. (I messed up and installed pro for a home key, so I did it back to back with same ISO)
You already have to disconnect form the network to be able to use the "offline account" option in the installer (last I checked, which was like a couple of months ago).
So this has been going on for a while.
Edit (merge the other comment I deleted):
To clarify, this was a Home Edition. I understand that the Pro edition doesn't behave this way.