"This is not a drill" makes me think there's an immediate threat that needs full attention, especially from snowden's account. Is this just more twitter slang to add emphasis?
You have to additionally consider that Snowden resides in a country under a "managed democracy" (to be charitable), and widely believed to be currently undergoing preparations for war. He probably expects his communication freedoms to be curtailed very, very shortly. To him it's definitely not a drill.
Definitely Twitter slang to add emphasis. "This is not a drill" implies that people have been preparing a response and now is the time to execute it properly.
