Status of a sandhi "rule" for 2nd tone
I asked the author of the blog eastasiastudent.net about it and he told me it was probably a mistake. Given the comments from @fefe and @JasonSwift, my conclusion is that the author of the first link was trying to describe a real phenomenon in spoken Chinese (i.e. the phenomenon described in the accepted answer of the referenced SE question) but made a few mistakes in the description.