Huh? I'm not referring to setjmp at all... (why would you think I'm referring to setjmp?)

The PNG format has lots of variations and optional bits that would be quite annoying if you needed to handle them all yourself. I'm referring to the many various flags, options, and little helper functions in libpng that make handling all these format variations quite painless for the user of libpng.

[Some libraries, e.g. libtiff, provide mechanisms to abstract out the low-level details, but still expect the application to explicitly handle many format variations, and this is quite annoying.]

libpng is hardly perfect; it's old, and that certainly shows in various ways (e.g. the use of setjmp). But to call it the "worst API ever" is just wrong.