Earlier quoted context omitted.
Have you ever implemented GC or fibers? They can be pretty simple to implement, really.
There is a big difference between implementing them for fun and implementing them for a production system. Regardless, even if they are simple to implement they are more than what C90 has so, by definition, more complex implementation.
They're a lot simpler than printf, strxfrm, or mktime, all of which are in C90!