100 psi means there's 100 pounds of pressure on each square inch! Woah! Think of gluing a patch over that cut and having it hold with all that stress trying to pull the cut open, while you ride over bumps and potholes, too.
The fabric casing is all that keeps the tire from blowing up like a balloon, and a cut interferes with that.

Tyvek scraps
I wrap my spare tube with a piece of tyvek from an overnight envelope. (or an event ride jersey number). It should make a nice boot, bigger and stronger than a dollar bill. I've only had to use dollar bills in the past, the tyvek has been waiting for a cut tire, for a couple of years now.

But the main reason for the tyvek is that tubes often stay in saddle bags for a long time, and abrasion weakens them at the folded edge. I've seen two or three tubes go flat soon after replacing a flat tube, and the cut isn't sharp, it's kind of worn down looking.

Rule of thumb
Big cuts only happen on new tires. Why is that