Remark that the article is rather old (although revisited 2014, however that's still an eternity in webdev world). Although I like flask (it was my first library to do "web stuff") I miss especially the fact that authentication is not part of the core lib, but only available via third-party extensions.

That may not be a problem for an experienced dev, but for a starter in web-dev I highly appreciate a library or framework that has built-in authentication because you do not want to fuck that up. Granted - you can also fuck it up with e.g. Django, but it is way harder.