What happens is the nose on the mainshaft has a bearing to the inside of the input gear.
There is not enough oil to allow that bearing (and surfaces) to survive a long 60MPH jaunt in neutral.
It used to be, when folks had manual transmissions, if you were taught at all, you were taught NEVER to put the transmission into neutral and coast down a long mountain grade.
On the two that destroyed the tunnel and such....
If that bearing seized, it is no longer in neutral, rather 4th gear, and you can destroy everything else.
Just because someone got away with it once, doesn't mean you will.
Someone might get away with a zing up to 90MPH once, but you try it, you get pulled over.
You know how that works.