There are cryptographically secure pseudorandom generators. In fact all sources non-hardware (i.e. /dev/random and /dev/urandom) are psuedo random as are the keystream generators for most algos that use one.

Yes, I'm aware of that. The commenter seems to be implying that one should stop using a shell script when one needs "true" random, and I'm curious as to why since one can call out to Perl, Python, etc.

The linked gist doesn't appear to be making the case for using its recommendations in cryptographic applications so what I'm ultimately after is what the commenter thinks is being done in the shell that's inappropriate.

Incorrect. I'm not sure what you mean by "true random", and we could get into some interesting philosophical discussion on that.

However, all software random number generators are pseudo random, including cryptographic ones. They are called CSPRNGs, which stands for Cryptographically Secure Pseudo Random Number Generator.