He mentions multiple times in the first few paragraphs that this is an evolution of the current bcache code:
"For those who haven't kept up with bcache, the bcache codebase has been evolving/metastasizing into a full blown, general purpose posix filesystem... I and the other people working on bcache realized that what we were working on was, almost by accident, a good chunk of the functionality of a full blown filesystem - and there was a really clean and elegant design to be had there if we took it and ran with it."
"For those who haven't kept up with bcache, the bcache codebase has been evolving/metastasizing into a full blown, general purpose posix filesystem... I and the other people working on bcache realized that what we were working on was, almost by accident, a good chunk of the functionality of a full blown filesystem - and there was a really clean and elegant design to be had there if we took it and ran with it."