It's a bit misleading to use "build time" to describe this improvement, as it makes people think about build systems, compilers, header files, or cache. On the other hand, the alternative title is descriptive and helpful to all developers, not only just builders - people who simply need to clone a branch from a large repository can benefit from this tip as well.
"This simple one line change reduced our clone times by 99% and significantly reduced our build times as a result."
So the title is just completely wrong.
"We found that setting the refspec option during git fetch reduced our build times by 99%."
So, the article contains contradictions.
Where in the article does it say that? The article says this:
> This simple one line change reduced our clone times by 99% and significantly reduced our build times as a result. Cloning our largest repo, Pinboard went from 40 minutes to 30 seconds.
Both of those sentences say the clone time was reduced by 99%. There are percentage numbers given for how much the build time was reduced, nor any numbers about the total build time.
Presumably the build does more than just clone
If the title is a lie (which it probably is), then nevermind that number, but it's clear where it came from.
The only way build time could have been reduced by 99% is if every part of the build other than cloning is negligible. It is far more plausible to assume that the title is simply wrong.