I asked you, point blank, HOW you, the hyperintelligent AI, would threaten the human race.
Uh, what did you expect? A step-by-step guide to world domination by the hyperintelligent AI I keep in my calculator?
The military thing was about how a malicious AI could come to be. But sure, the airforce would be fucked without computers. If I have control over the aircraft, what's stopping me from shutting down their engines in midflight? Or just make them crash into something by using the fly-by-wire system? Fighter crafts especially are much too unstable at high speeds to be flown manually. Not to mention the fact that as an AI you'd also have taken over the Anti-aircraft weapons.
"Lying in the press" is something completely different. A "hyperintelligent" AI should be able to subtly influence media coverage to get a desired effect. It's hyperintelligent. Like a supercomputer can compute many more moves of chess ahead of you, our hypothetical AI would be able to calculate every twitter users' reaction to the "bunch of fake tweets". Again, it's not some random jerk spamming. It's an intellect much, much more advanced than anything in existence right now.
Who says I wouldn't take over a factory for industrial robots? And raw materials can always be ordered. I can change the supply chain. After all, I'm the one in control of all the logistics software.
I actually doubt that I'd be able to infect a computer in a nuclear plant with something as primitive as a USB stick, but on the off chance that it did work, I'd be easy to fake read-outs to the controllers (via the SCADA). Alternatively, you could just uncouple the turbines and watch them blow. Suddenly your power has nowhere to go and you need to initiate emergency shutdown. Even if I can't do anything, I can decieve the controllers. If I do this to all plants, it will succeed in at least a few.
And I'm not sure where you get the idea that shutting off every power plant would be a "brown-out". It would be a blackout. The grid would be useless because there is nowhere for the power to come from, not to mention that the grid itself is hugely dependent on computers to balance load. It would porbably be easier to attack the grid, actually. It's a commonly explored scenario, and if we even now think human hackers could pull it off, an AI could easily do it.
Not to mention the fact that an AI would only have to snip its fingers to destroy the economy. High-frequency trading spiralling out of control would wipe out massive amounts of money in an eyeblink. Even if trading is halted immediately, I can do it again. And again.
Even just shutting down the internet would cause massive damage.
Society relies on computers to a massive extent. You can deny that as much as you want, it won't make it less true. Even if it isn't, an AI could just pretend to be friendly until we rely on it. Or it could stay hidden, waiting for the time when there are millions of cars ready to drive over everything that moves.
The problem is not a single one of these things. The problem is all the things, at once. An AI can pull this off. An AI can also think of stuff we can't think of, by definition of being smarter. So getting to the T-1000 with his shotgun is just a natural conclusion. Except that killer-robots are way too inefficient for any killer-AI worth its silicon: Much easier to release a virus. Instead of autonomous robots you can always hire people willing to commit crimes. Like, say, break into the CDC.