I'm not sure what their answer means. They used a similar hand wavy answer about why the language has null pointers. "Complication" here refers to how complicated their implementation of the compiler is, not the end user complication, which is what they traded off (simpler compiler for more complex user code).
I didn't get the last point. builtin functions are there (for a big part) because the language doesn't have generics. Functions on types can be added without affecting the interface, since strings don't implement any interface (at least not on purpose, another problem with golang). Java constantly enriches standard library types with more useful methods.