"bash only" typically refers to "bashisms", that is, bash features not present in the plain Bourne shell (or Bourne-compatible interpreters such as dash). The fact that other shells (such as zsh) may include those features ... is beside the point of writing universally compatible shell scripts.

Confirming my facts for this comment, #TIL that "dash" is the "Debian Alquist Shell", that is, Debian's "ash" shell:

<https://en.wikibooks.org/wiki/Guide_to_Unix/Explanations/Cho...>

And ksh (ksh88 & later)