A fire axe works fine when you're in the same room with the AI. The presumption is the AI has figured out how to keep people out of its horcrux rooms when there isn't enough redundancy.

However the trouble with late game AI is it will figure out how to rewrite its own code, including eliminating kill switches.

A simple proof-of-concept example is explained in the Bobiverse: Book one We Are Legion (We Are Bob) ...and also in Neil Stephenson's Snow Crash; though in that case Hiro, a human, manipulates basilisk data without interacting with it directly.

Also as XKCD points out, long before this becomes an issue, we'll have to face human warlords with AI-controlled killer robot armies, and they will control the kill switch or remove it entirely.