It looks like you start dragging one, far enough away from its initial position that you are unambiguously dragging it, then (while maintaining contact with the dragged icon), tap the other icons you want to drag as well.
Thanks, I remember doing this once, but before I wrote the comment spent 5 minutes trying to do it again and couldn't figure it out. An interface that a long-term user can't easily discover is kind of terrible; it's also completely different from how a desktop OS works where you select first and then drag.
