If you're keen, you can get the source code and take a look: http://www.arclanguage.com/install

File system stores aren't that uncommon (aside from the VSAM days, even in RDBMS times+) - it's a common approach for Wiki implementations too.

Also, unless your DB tables are in-memory tables, you are using a very fancy interface to a file system store (that is usually in a nontrivial format.)