He's not wrong either. That UI is older than Windows 95 (which stole and misused some of its look and UI elements, that is an iconify control dammit) but it still looks fresh, simple, and get-the-job-done. As well as ignoring the "flat" trend and the "pretend to be a touch UI" trend.
The "Maximize" icon in Windows is clearly a stylized depiction of a window with a toolbar, sized to fit the button on which it is. I don't know if it's the perfect icon for this action, but it makes a lot of sense. Why should it stand for iconification, even if Windows still had that feature (last time it did was Win 3.11)
In the context of Nextspace, Nextstep, or Windowmaker, it stands for Iconificaiton because it is "clearly a stylized depiction of" the icon that appears when a window is iconified.
Compare the iconify button with the icons representing iconified windows below: