The decentralized web works by having a p2p distribution of the files that make up the website, and then the website runs in your browser. By being completely portable, the website has all the pieces it needs: text, programs, and data. It can all be versioned, archived, and examined.
He mentions IPFS in particular, which I’ve written about before. The bottom line is that static HTML sites are more portable. They more naturally evolve not just from host to host as necessary, but also to a possible distributed future web. That’s why that — even though I still use and recommend WordPress — I have a static mirror of my site too.