You are exactly right -- it's not the Foundry Image stack.
But consider this: Reading this thread, you can get the impression that it is not possible for a site using Foundry to support images for retina displays. That's what I thought 10 days ago when I came across it while deciding whether to buy Foundry. I read it, assumed that I would have no options regarding retina images when using Foundry, and I decided not to buy Foundry.
I then bought a theme by a well-known designer who will remain unnamed, found a project-killer bug in it, bought a theme by a 2nd designer and found multiple project-killer bugs in it. (Off-topic: What has happened to the quality of RapidWeaver themes?) Then I read Isaiah's statement that I quoted above, which shed new light on this thread, and I bought Foundry.
Everyone else who has contributed to this thread has more experience and expertise with Foundry than I do. And I'm as confused as anybody on what to do regarding images for retina displays. When I read this thread the first time, I thought that if I used Foundry, I'd have no option other than to forget retina displays exist. But based on Isaiah's statement, I don’t think that’s true.