I think a good threshold, and definition, is when you get to the point where all the different, reasonable, criteria are met, and when saying "that's not AGI" becomes the unreasonable perspective.

> how could it possibly be achieved?

This doesn't matter, and doesn't follow the history of innovation, in the slightest. New things don't come from "this is how we will achieve this", otherwise they would be known things. Progress comes from "we think this is the right way to go, let's try to prove it is", try, then iterate with the result. That's the whole foundation of engineering and science.