The guy in the you tube had no idea what he was looking at. Shoddy workmanship from the axle maker.
The damaged wire shown in the last few second wasn't caused from laying in a steel axle tube but rather when the wires at assembly were jerked through those small holes. Those sharp edges scrapped the insulation off and even cut strands in the copper wiring .
I saw this when working at a mobilehome mfg in the early '60s when final assy line electricians did their brake wire testing.
We pulled lots of shredded/scuffed wiring out of the axle and ran our own wiring after redrilling the hole at a angle with a plastic grommets for that purpose
I would never put trailer brake wiring on the outside of a axle tube.
