That assertion is disingenuous at best.

That guarantee is only achievable at the expense of forcing compiler developers to maintain a GUI toolkig for all platforms. Who in their right mind believes that's reasonable or desirable?

Everyone that wants a language to thrive instead of dealing with thousand incompatible implementations.

Many C++ targets don't support IO or networking, so lets not burden embedded compiler developers with standard library bloat.

So instead of a GUI library what about a HTTP or a network library as part of the standard? Surely handling TCP and UDP connections is an order of magnitude easier to implement and maintain.