I too immediately thought this statement was a misstep in Greenwald's otherwise discriminating journalism. The possibilities you outlined detail exactly why, but I would add a fourth, if somewhat far-fetched, circumstance to your list.
(4) Snowden has verified knowledge of an imminent death threat from the USG and is using Greenwald to convey awareness of his "dead man's switch" for his own protection. Employing Greenwald as a channel for this message, as opposed to himself, gives him some cover legally and won't further alienate Snowden from the public who might see him as blackmailing the United States if he were to outright state this in say, a new video interview.
(4) Snowden has verified knowledge of an imminent death threat from the USG and is using Greenwald to convey awareness of his "dead man's switch" for his own protection. Employing Greenwald as a channel for this message, as opposed to himself, gives him some cover legally and won't further alienate Snowden from the public who might see him as blackmailing the United States if he were to outright state this in say, a new video interview.