Worse, they didn't actually get a 15% speedup. They reduced the frame time from 16.7ms (around 60FPS) to 15.3ms (around 65 FPS), which is only a speedup of something like 7%.

They didn't seem to mention how much they're actually utilizing the remaining cores either. From the look of it, they've basically parallelized a couple of loops in the renderer. That strikes me as being the wrong way to go about this, especially considering that modern AAA games apparently break everything down into tasks, and run the separate tasks in parallel (idTech5 apparently does this, for example).