I watched a presentation of these guys at SHA2017 and someone asked the same thing. Here's the full presentation in case you're interested: https://media.ccc.de/v/SHA2017-120-hack_north_korea
As far as I recall their answer was that a USB drive can store much more. Video, entire books, etc. However leaflets are also done.
Personally I don't know if it's worth the risk knowing that their local OS ("Red Star OS") can fingerprint files. But I guess they know what they're doing.