At first, I could not figure out why sleep mode was an issue, but I think the point is that a cold boot attack has to be performed within minutes of the shutdown, and it has to be a 'hard' (just cut the power) type of shutdown, not an orderly shutdown where the OS stops what's running and then instructs the hardware to shut down. An attacker who gets his hands on a computer in sleep mode is in a position to force a hard shutdown and immediate cold boot when he is ready.