Plain text is not superior, merely more well-supported because the libraries for it have been around longer, and standards for it had been created. If adequate standards for WYSIWYIG were created, it would be definitely better than plain text from an end-user point of view.
In fact, there are numerous cognitive drawbacks that can by corrected by using secondary notation (available either in WYSIWYG and markup), so unadorned plain text is objectively worse than rich text - if only good tools existed for the latter; the author is right in wanting hackers to make those ubiquitous.
In fact, there are numerous cognitive drawbacks that can by corrected by using secondary notation (available either in WYSIWYG and markup), so unadorned plain text is objectively worse than rich text - if only good tools existed for the latter; the author is right in wanting hackers to make those ubiquitous.