The example did not have threaded code so I find your nitpick unfair. The usage of random in their examples is perfectly fine.
If "idioms" mean code that adheres to best practices, given that C# code is frequently threaded, their examples should mitigate common threading issues.
It will be ignored. StackOverflow guys and all participants have put an enormous effort into making it at least partially trusted source of snippets, and I hope it will never be outranked by sites like this.
(to creator: sorry for this, but in a long run it only harms everyone, unless you “die” for it)