They already wrote

> The tests would have been a real pain to write in C++

so i guess what the script piece above shows wasn't the entire deal. When you have a custom language you have full control over its syntax you can make it do all sorts of things that are hard to do in something like C++.

As an example you can build a tree of key-value nodes using C++ calls just fine, but writing a JSON file is usually much simpler and concise (especially when deep nesting comes into place). And a custom syntax could be even simpler than JSON.