That's like saying a well-written C program is a good reference for C++. In any case, writing POSIX-only shell scripts is much harder than writing Bash scripts, which can often be much faster (e.g., one can use {1..10}, which doesn't need an external call, instead of `seq 1 10` to iterate through a list of numbers). For POSIX-only shell scripts, the ones used in Git [1] are some of the best-written I've seen.
[1]: https://github.com/git/git/search?l=shell