As one who's actually had the pleasure of getting to use one of the classified drive destroyers, there aren't armed guards. Not sure if there's a requirement that it be done in a room with no windows, though.

My point was that Kenny's claim is that on-drive encryption allows for easier "erasure", presumably by deleting the keys needed to decrypt the data and leaving the drive in the same state.

If that process is good enough to ensure proper "erasure" of the data (effectively complete prevention of data access), then how is it not equally secure as an encryption schema?