What is the association with Mahler?
Justin,
I think you've answered your own question with your follow-up post: the various typefaces in the Ionarts article either come from Mahler's fin-de-siècle period or were designed to appear to have come from that era. I've also seen era-specific typefaces used with the music of Arnold Schoenberg and Alexander Zemlinsky.
Regarding Rennie Mackintosh, note that there are design differences among the various fonts within the Rennie Mackintosh typeface, e.g., Standard, Light, Bold, Italic, etc. The Ashkenazy Mahler 9 cover includes both Bold and Standard, while the "Gustav Mahler" printed below it is in Light.
Yes, it was after I asked the question I googled "Mahler font" and that Ionarts blog popped up. Now I know how the graphic artists making these album covers choose their fonts!