I don’t have a dog in the fight, but I would think any real db would be a disqualified in the spirit of flat files. But, SQLite honestly seems like the best of both worlds to me.
no, SQLite was built to be a flat file db. Its quite exactly the best solution if you need a db but want to have it contained in a folder with your other files.
in an URL and expect a user's browser to display something sensible like it would with an HTML file (or even a fixed-format text data file). You need a process on your server that translates the content in the database to browser-readable HTML.
So in the sense of "put up a website by just transferring static files", an Sqlite database isn't quite "a flat file". It's a single file, yes; but its content is "bumpy" -- not "flat".
(Then again, once you have that translate-content-to-HTML process installed on your Web server, updating the content is reduced to just transferring a new .db file to it.)