They said they contacted the developers. They're both known people so I guess the developer trusted them enough.
So, in other words:
Yogscast: We heard about this game and we want to play it before anyone else can.
Devs: Suck our richard and you can have it for free.
That's basically what happens. Only famous richard suckers on YouTube can get "special privileges" to play games just to make it famous. It's the most disgusting defect of the human mind. Get famous. Get money. Be an ass.
I'm not saying the game is bad. It's just how unfair things are.