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)