This diagram is to answer the private blockchain hype. It's not meant to address any piece of technology- just qualify when a blockchain doesn't provide clear value.

With the same logic also a BitTorrent tracker should use a blockchain, but obviously it's not the case. You are distributing trust-based units (money, legal and financial instruments, etc), not files (video, images, etc).

I think very many IPFS use cases do need / benefit from blockchains.