I think the point is that a full-screen window that's largely empty of UI means that you don't see other windows, icons, toolbars etc. that are the potential sources of distraction.

It's not that the WP is distracting. It's that the WP leaves other sources of distraction -- such as playing around with formatting, other apps and so on -- visible.

A WP with little or no onscreen UI and which runs full screen so you can't see anything but the WP is trying to minimise visual distraction.