I'm guessing here, but I can see the mixing simply going from one of these masternodes to another, ultimately allowing for many rounds in milliseconds. There is no time constraint like waiting for a block. The MNs do all the work, and simply send them to the miners for inclusion.
This is a great point and I think you're on to something, but that alone doesn't allow Darksend to be instant. You would still have the problem of waiting for funds to mix with. I think Evan must have something planned that's way outside the box!