Further and farther are interchangeable enough that it doesn't matter. There are usage guides on the internet, but they are not infallible by any stretch; the beauty of language, especially that of English, is that it is ever-evolving.
As for how these things get by testing, that's an easy answer - the terminology used in your example is an acceptable one. How ACTUAL mistakes make it through, however, is that they're too busy making a game to worry about necessarily always being perfect in every tiny detail. Copy mistakes are easy to make, often go through many revisions, pass many hands, and each step of the way is a human prone to error.