Maciej's larger point is that the AI faces tons of very difficult problems in escaping its physical constraints. It's simplistic to wave hands and say "the AI is super duper smart and will have no difficulty hacking all computer systems, inventing and manufacturing swarms of unbeatable nanobots, etc. without being detected or resisted".
It seems to me that the core conceit of AI doomerism is that sufficient intelligence can overcome all barriers with some plan that is so smart, people would never think of it. This is much less plausible than believers take it to be. In mathematics alone, it is very easy to come up with a problem that the collective mathematical ingenuity of the entire human race is helpless before for decades, centuries, or longer.