Linux doesn't dictate a native API for GUI. It de facto dictates the display server protocol though (currently being X11 and Wayland as the next generation). So any API would be built with that in mind. That said, most widely used APIs are Qt and GTK+.

That's what I thought. So I'm having trouble understanding how I would apply bitwize's iron rule.

Ever noticed how Linux appspace tends to be a horrible mishmash of different GUI styles? That's why Mac users are nazis when it comes to obeying the Apple Human Interface Guidelines - the alternative is every developer does whatever he thinks looks good, and there's no consistency to your desktop. OS and UI consistency is a problem if not done well.

I strongly doubt he ever used his own rule while doing cross platform development. Or he'd know better.