Brake Line Fabrication Attempt and Seized Nut Failure

What happens here

The presenter prepares to work on the brake lines by laying a piece of plywood on the ground to slide under the truck. He then shows a section of the old brake line that broke off, leading from the master cylinder to the proportioning valve. He points out the extreme rust on the old line. Since the local auto parts store didn't have a pre-made replacement, he plans to custom-fabricate one using bulk tubing. He demonstrates cutting a new brake line with a tubing cutter and installing a new nut onto it, explaining that the next step would be to flatten and flare the end of the line. He then goes under the truck, using a Kobalt 9/16-inch flare nut wrench to attempt to loosen a seized nut on the brake block. Despite his efforts, the nut is too rusted and becomes rounded off.

Goal: Replace the broken brake line between the master cylinder and the proportioning valve.