That's not the point. We can map any multi-dimensional form of intelligence into a single dimension with any suitable norm. It just matters that for any dimension we can quantify intelligence in the sense that something more intelligent is imaginable.
> Humans do not have general purpose minds, and neither will AIs.
That does not matter at all. Then a superhuman intelligence is not "general", so what? It's still wastly more capable then any human.
> Emulation of human thinking in other media will be constrained by cost.
But what is the cost constraints? Human brains are constrained by mechanical considerations. How efficient could a computerized AI become?
> Dimensions of intelligence are not infinite.
Again, what's the argument? The number of atoms in the universe is not infinite, either (probably).
> Intelligences are only one factor in progress.
That's not the problem of the super AI.