They were ruled out (with some probability of error) theoretically once people already had working designs, and using knowledge obtained from experimentation on smaller designs.

Nobody here is suggesting to wire the first experimental AGI to the nuclear missiles launch systems. The point is that you need a good idea about what a working AGI design will look like before you can say anything meaningful about its safety.
Experimentation, with reasonable safety measures, will be most likely needed before a full-fledged design can be produced.