The "more to it" is "if the AI is much faster at thinking than humans, then even humans in the observe/decide/act are not secure". AI systems having bugs also imply that protections placed on AI systems would also have bugs.
The fear is that maybe there's no such thing as a "superintelligence proof" system, when the human component is no longer secure.
Note that I don't completely buy into the threat of superintelligence either, but on a different issue. I do believe that it is a problem worthy of consideration, but I think recursive self-improvement is more likely to be on manageable time scales, or at least on time scales slow enough that we can begin substantially ramping up worries about it before it's likely.
Edit: Ah! I see your point about circularity now.
Most of the vectors of attack I've been naming are the more obvious ones. But the fear is that, for a superintelligent being perhaps anything is a vector. Perhaps it can manufacture nanobots independent of a biolab (do we somehow have universal surveillance of every possible place that has proteins?), perhaps it uses mundane household tools to macguyver up a robot army (do we ban all household tools?). Yes, in some sense it's an argument from ignorance, but I find it implausible that every attack vector has been covered.
Also, there are two separate points I want to make, first of all, there's going to be a difference between 'secure enough to defend against human attacks' and 'secure enough to defend against superintelligent attacks'. You are right in that the former is important, but it's not so clear to me that the latter is achievable, or that it wouldn't be cheaper to investigate AI safety rather than upgrade everything from human secure to super AI secure.