Why, aren't GPUs pretty useful in training SVMs as well?
Well, and though the author shows an approximate equivalence, which helps us understand a class of models better, it's not obvious that it's preferable to use SVMs of the type described. In particular, it seems like often it would be preferable to deal with model weights (even if they are mathematically a "superposition" of datapoints) than to ship around and revisit the whole dataset.
SVMs are solved via convex optimization methods which have taken more time to get on the GPU train.