What some people are thinking of in simple terms is the speed of sound at sea level...it is FAR lower in the upper atmosphere as the air density is much lower. Speed of sound is easier to achieve at high altitudes.
So what is the speed of sound at 100,000 feet?
Even the good Captain (retired COL) never claimed to have gone through the sound barrier.
I forgot to add in my last post to you that they, and he, noted that he would have done it if he had gone a little over 4,000 feet higher.