I'm not sure there's anything "impossible" about how small this is. You don't really need a lot of lines of code to support routes, request and response and nothing else. If anything, 765 lines of code for this is quite a lot.
It also uses libraries for most "extensions" that are available, defeating the purpose and bending the claim that it's 1,700 lines of code including the extensions. Just jinja, one of the dependencies, is 18,000 lines of code. If that counts my Nanodot server which calls flask.app.run() is one line...