I don't think this leak shows that they use Elixir. It's open-source code that Google customers are supposed to be able to use to interact with Google APIs. It's natural to provide support for your customers to use the languages they want to use, rather than the ones you've settled on internally; this is important for anyone, let alone restrictive internal environments like Google's is supposed to be. (https://aws.amazon.com/sdk-for-php/ exists, but Amazon famously doesn't allow services to be developed in PHP.) Google is famous for allowing only a few languages for production code.

The README also says:

> Disclaimer

> This is not an officially supported Google product.

which, while it doesn't directly indicate that Elixir isn't used internally to Google, would be a surprising mismatch of support if they did use Elixir. Here are their official clients (including some in languages that I doubt they want to be used internally for service development, like PHP, Node.js, and .NET): https://developers.google.com/api-client-library