Just because there isn't a specific isometric subforum doesn't mean there is no place for it at all. There isn't a network forum as well, yet network-specific topics do exist on ~~gamedev.net~~ devmaster.net (see http://www.devmaster.net/forums/showthread.php?t=5818 for a recent example). So your argument proves nothing.
Isometric questions will be perfectly fine in the "Graphics theory & programming" subforum, no one claims that that subforum is solely for 3d graphics (as a matter of fact, the description line reads: "This forum is for discussion of all aspects of 2D and 3D graphics theory and programming").
But I see at least two people interested in isometrics (you and the topicstarter). If an isometric subforum is to be created, it'll need to be filled with content as well; questions don't come by themselves. So I suggest you two start off with an interesting discussion/question regarding isometrics in Graphics Theory & Programming