How can a prohibition on "text-based browsers" have anything to do with MitM attacks? The browser's chosen rendering method (text or GUI) is irrelevant.

Google is actually misrepresenting the danger in their blogpost, because they write "One form of phishing, known as man-in-the-middle,..." - this statement is clearly false because MITM is not phishing. Not in any stretch of the definition. So who knows what is the real justification.