Common usage, which you can see as easily as I can. For example, do news agencies typically use FBI or F.B.I.? Do they typically use UN or U.N.?
> That is, what is the evidence that current professional editors are on record as saying that the quoted advice is old-fashioned and not to be regarded?
You again misunderstand the role professional editors (and, by extension, style guides) fill. They enforce what's commonly known as a 'house style', which may be influential outside of a given 'house' (company or newsroom) but is not authoritative in any sense anywhere but that single location. Language is not defined by books: it is described by them.
> (This subthread started, after all, with an HN participant noting that editors were FOLLOWING the quoted rule.)
That comment was deleted before I got to it.