Reproducible builds generally work from the published source tarballs, as those tend to be easier to mirror and archive than a Git repository is. The GPG-signed source tarball includes all of the code to build the exploit.
The Git repository does not include the code to build the backdoor (though it does include the actual backdoor itself, the binary "test file", it's simply disused).
Verifying that the tarball and Git repository match would be neat, but is not a focus of any existing reproducible build project that I know of. It probably should be, but quite a number of projects have legitimate differences in their tarballs, often pre-compiling things like autotools-based configure scripts and man pages so that you can have a relaxed ./configure && make && make install
build without having to hunt down all of the necessary generators.
Unfortunately, it's definitively an instance of intentional design. This whole consent dialog thing became a booming "consent management platform" industry. Many of them advertise better acceptance rates than the competition, or used to but have removed those claims in more recent times now that the big GDPR boom is over.
This particular dialog is TrustArc, who are infamous. At one point they defended it with a "well, we gotta retry if it fails to make sure your preference is expected, and we can't know if your adblocker is causing it to fail or if it's just a fluke", which is one of those things where they say something that's not totally wrong but you know they're lying through their teeth.