You kinda missed the key points of his post. Haste still increases the scaling of DS even after hitting the global cap, albeit to a far lesser extent, by increasing the rate at which you gain fury, and decreasing the time between Metas. Second, haste will always affect SF scaling in both senses; increased casting speed and decreased time between Metas (assuming you're using it as a filler). It also scales better with int than DS does, meaning there will be some inflection point at which SF is always the filler of choice while in Meta. Part of that scaling also depends on MC procs.
It's also helpful to realize that the costs of the spells aren't finalized, either. They might be close, but shave a little fury cost off of SF, and suddenly it becomes worth more than DS, even now. Nerf the scaling a little on DS; same story.
My point is there's nothing wrong with the model I'm describing. It might not practical as of this build, but then again, nothing is final yet.