Honestly, I don't read FT for a couple of days and miss 75 posts on my special subject!

The website above is just another style guide, and IMHO a rather simplistic one. For words, yes, apostrophes should be used for possessives and not for plurals, or for missing letters. But for abbreviations (although it's stylistic and a matter of personal preference) it is possible to use them for plurals, especially where this results in greater clarity. In this usage, the purpose of the apostrophe is to mark the end of the abbreviation.

I.e. If you talk about 777's or CD's, it makes it clear that the abbreviation in each case is 777 and CD. Instead, you might think the "s" is part of the abbreviation. If this seems contrived, think of car names, where you often have an s in the model number. So if you just said "She ordered the 940ds", you wouldn't know if a single 940ds or multiple 940d vehicles were involved. This is an instance where non standard use use of punctuation still fulfils a valid, disambiguating purpose.