IMO, the delay between spraying and disking is more to allow the grass and weeds time to decay so that disking and planting becomes much easier. Otherwise, you're constantly having to clean the grass out of your disk.
Whatever the spray hits is as good as dead within a day or two.
My regime is to mow the plot first, and then give it a couple of weeks to begin to grow back and allow time for the cuttings to decay. This ensures the leaves of the still growing grass and weeds will catch plenty of the spray, which is necessary for an effective kill.
A liitle liquid dish soap added to the spray mix helps the spray stay on the leaves better for a more effective kill as well.