Note that Linux, today, is far from the largest Git project. Oldest, yes. Biggest, not even close. FAANG monorepos are vastly bigger with much higher commit rates. The Linux kernel is quaint, small, and moves glacially slow in comparison. Even if we consider only public open source codebases, Chromium's repo is bigger than Linux by any reasonable metric.
Niche use cases are a great reason to make a bespoke tool! But that doesn’t make it not a niche use case. And it doesn’t mean that tool is optimal for the common use case.
In the sense that it was the first project to use git (has been using git the longest), not in the sense that it's the oldest project that switched to git.
IIRC, the oldest was git itself (it self-hosted early on its development history); git, linux, and sparse are IIRC the three projects that have been using git the longest.
