This idea has been around for a while... BeOS allowed for indexing and metadata which allowed you to use the file system a lot like a relational database. It was a really cool feature and made it easy to write software that stored tabular data. MS tried back in the Windows Longhorn era... and gave up on it. I think there's more than one 70s and 80s era big iron OS that did this, too.
The file system has been really hard to get rid of - it hides a surprising lot of details on how bits get read and written from hardware, and that hardware has changed a lot in the past decades. SQLite is a great codebase to look at to see what really goes on there.
Filesystem is small aspect here, the big deal here is putting stuff like task scheduling and IPC on top of DBMS. And even for files, the point here is more on having distributed filesystem as first-class citizen and less on being able to do sql-like queries.
Although not the same as what the paper talks about, an approximation would be Oracle running with raw volumes managed by the DB, on a cluster, using Web based UIs done on Apex.
The file system has been really hard to get rid of - it hides a surprising lot of details on how bits get read and written from hardware, and that hardware has changed a lot in the past decades. SQLite is a great codebase to look at to see what really goes on there.