If it's about auth, then I'd say that's more about the "edges" / porcelain than the core. The claim about git is that you can re-implement the core data structures pretty easily, and they are stable
While new functionality can be implemented on top of this stable base. It doesn't surprise me that it's hard to re-implement all of git, including the network protocols, but that's a different thing than the core
This thread has some good info about the core, including the tree encoding issue brought up by bradfitz, but it still seems to be approximately true ... e.g. perhaps compared to hg
I also remember that JGit is a very complete re-implementation of git, done by a git core dev, and used in Android infrastructure, etc. This was well over 10 years ago at this point, maybe 15
(gitoxide is a similar project but in Rust, it's not ready for the big time either, though it keeps on getting better!)