As a user, I did not choose to use bashcompinit. It is the zsh
completion system that uses it, via a system-level completion.
This is not the case. There is no completion for qpdf distributed with zsh. You or your system installer have chosen to make it available from a third party (possibly the qpdf application itself), and the supplier of _qpdf has chosen to implement it by using the optional bashcompletion support rather than to re-implement it in native zsh.
The bashcompletion sources are specifically designed to implement the bash interface to completion definitions as closely as possible, so that any functions written for bash do not have to be rewritten, but zsh does not require or even encourage its use.