Their justification attributes a blog post where the author basically says "don't use tools that will break your code if you omit these syntax features". It encourages people to write code that will break when used by others, dictating the tools they can and can't use. It's the equivalent of a child closing their eyes and sticking their fingers in their ears to magically whisk away the person standing next to them.

It's pretty disappointing to see GitHub, a company that a lot of developers (including myself) look up to and will follow, espousing something like this.