The posters rebuttals against the threat are totally under-thought cop-outs. For example, his first argument about "how would Hawking get his cat in a cage?" just put food in it. It's not hard to imagine an AI could come up with a similar motivation to get humans to do what it wants.
That's not to say that his general premise is wrong, but it's hard for me to take it seriously when his rebuttals are this weak.
The emu "war" example is also similarly dumb. We know that humanity is very successful at wiping out animals even when we're not actively trying, just as a consequence of habitat encroachment or over-hunting. If you want to kill a bunch of emus, that can easily be done using appropriate methods. Having a cohesive military formation go after them in the huge Australian outback and giving up after a week is not the way to do so.
That's not to say that his general premise is wrong, but it's hard for me to take it seriously when his rebuttals are this weak.