Why use this instead of Slim? Slim is basically a fully-baked version of this (with a similar router), and it's decoupled, so you can use only the parts that you want.
They're not strange/crazy enough to make it a fun programming challenge, and for something that's intended for use (even non-production use) they make the code less usable.
"No namespaces" and "no new functions or classes" in particular don't make any sense. Anonymous functions are just like functions, except they're harder to identify in a stack trace.