Note: I am the author of this article.

Yes, Mozilla doesn’t publish the source code. Back when I was reviewing add-ons there (a long time ago), I did compile the supplied source and compared it with the submitted one. It was sometimes awkward when the tool in question didn’t produce reproducible builds but mostly ok.

Hopefully they try to automatically reproduce the build and reject when it fails:

https://reproducible-builds.org/

Another Mozilla requirement is reproducible builds. This enables the to verify that a build from the supplied sources matches the release bit-by-bit.