It's a mountain relative to its surroundings. Glaciers are effectively frozen rivers flowing very slowly downwards, and Icecrown is a giant glacier flowing down into Crystalsong and Scholazar
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I do have a pet theory that the Dragon Isles were meant to be west of Northrend, rather than east. And that they changed this during development for some reason.
It makes more sense in terms of proximity to the Nexus in Boreal Tundra, as well as Centuar migration from Kalimdor.
Maybe this passage in the book is an artifact of that.