The idea behind the byline is of course that, for me, there is only 'one true scale' for gaming, and that is 28mm. Except for planes, where it is 1:144. Oh, and WWII, where it is 20mm. Or 6mm. Bah - whatever :).
The very concept of there only being 'one true scale' to game in is of course ridiculous. Any scale of miniatures you have an enjoyable game with is the correct and one true scale, and there's nothing more to it than that.
That said, I came up with the byline because for my hobby, 28mm is the sweet spot. One of my main interests in this hobby is the visual aspect of it, and more particularly the (more or less) nicely painted toy soldiers we use.
In very broad terms, the things that attract people to miniature wargaming are in the name itself: miniatures, war (or rather, the history thereof) and gaming. Of those three, the 'miniatures' part is the main attraction for me -- I'd be playing historical board wargames otherwise. Painting miniatures (and terrain) is an essential part of the hobby for me. And I find that for the sort of painting I do, 28mm is the best. Hence the 'one true scale' bit.
As to the actual phrasing of the byline -- 'There ain't but one ...' -- that's a blatant rip off of a wonderful sketch by Stephen Fry and Hugh Laurie, which forms a perfect (and topical these days in Belgium) note to end this post on: