1000 lines of shell is about 900 lines too many for "simple".
This just sounds like dumb shell hate.
I personally would have used another language, but shell ain't that hard if you have some experience with it, and are using shellcheck as the author does.
The config formats of the distro are optimized for command line tools, and writing the package manager in shell seems like a good way to ensure that this is indeed the case - basic dogfooding.
Shell is great for scripting usage of other programs, far better than other programming languages. That's what it's made for, after all. It's the only thing it's made for.