Right, but the goal isn't to make a strong or useful encryption tool. The goal is to show that cryptography is, at it's core, rather simple. It's simple enough that we can cram "real" symmetric encryption into the size of a tweet.

It's like building an electric motor out of some scrap wire and some fridge magnets - the motor isn't really useful for anything, but it teaches the basic idea in a way that anybody can appreciate. A useful motor would have bearings, a commutator, multiple windings, an iron core, etc., but adding all that stuff just obscures the basics. Criticizing a school-kid's motor for being weak and inefficient is completely missing the point.

This program isn't supposed to be useful. It's supposed to prove a point.