Sorry, but nine-hundred and ninety-nine times out of a thousand, it's a complete and total waste of resources to write a proprietary language or runtime enviroment to solve a non-core business problem.
First: they didn't write a runtime. They used .NET/Mono as their runtime.
Second, you wrote your comment in a text box rendered by an application notorious for being written in a custom language (in fact, it seems like arc:lisp::wasabi:.net).
Third, do you have evidence to support "nine hundred ninety-nine times out of a thousand", or is that hyperbole? Have you worked on projects where people used custom languages? Your argument would be much more interesting if we could read more about your experience with it.
You could view their core business problem as easily creating products that as many people as possible can pay for (since that's how they make their money).
