Earlier quoted context omitted.
They're using git as a cache. Having generated files stored there means they're available if they're needed (eg in CI) without needing further access controls, they're versioned, and it's a simple and understandable strategy. As the article states, most devs are set up to ignore those files so they're not much of a source of the slowness. It's a common pattern for apps that have to serve lots of different locales.
It's still surprising to have any generated things there. E.g. you could make the same case for keeping built binaries in Git as well. Is there a reason why that type of file couldn't be better place into an artifact repository, or just generated and consumed in CI as part of generating a final build output?
This is not surprising at all. In fact, it's quite standard to commit string translations. Just because you can run the code generation/string replacement step as part of the build that does not mean it's a good idea to generate everything from scratch at every single build.
String translations hardly change once they are introduced, running the build step takes significant amounts of time, and if anything fails then your product can break in critical and hard to notice ways.