the initial recursive cascade of an intelligence explosion can't race through human brains because human brains are not modifiable until the AI is already superintelligent

I am extremely sceptical about whether we will see much modification of human brains by superintelligent agents. Once we have superintelligence, human brains will go out of fashion the way the horse-and-cart did.

Brains will not suddenly become an attractive platform for future development with the advent of superintelligence - rather they will become even more evidently obsolete junk.