"Con-" from "concrete" is indeed the same prefix as the "con-" from "continuous" and in both cases it means "together".
However, "-crete" from "concrete" is not the same suffix as the "-crete" from "discrete".
"-crete" from "concrete" means "grown" (it is the participle of the Latin verb "crescere", to grow), so "concrete" means "grown together".
"-crete" from "discrete" means "sifted" (it is the participle of the Latin verb "cernere", to pass through a sieve, to separate), so "discrete" means "sifted away", it comes from the same source as the verb "to discern".
If I were you, I would send an email to Knuth reporting the error. You may receive a reward check from Knuth himself, which will give you eternal bragging rights.
However, not all his readers have understood the pun.
> So the main difference is that you can shape it without chiselling it.
I guess, yeah. Both are really good at withstanding compressive forces. Unlike Roman times, concrete today is usually reinforced with steel rebar, which gives it good tensile strength as well. This makes it useful for a lot of things that stone is not.
Disclaimer, take everything here with a grain of salt. Continuous-discrete is not the etymology of concrete. This might help too: https://youtu.be/UOHURuAf5iY