Bruce Schneier has said something similar, having seen some of the guardian documents. https://www.schneier.com/blog/archives/2013/09/the_nsas_cryp...
EDIT: Ah, it appears to be confirmed at http://www.huffingtonpost.com/2013/06/10/edward-snowden-glen... ... Thanks!
Second paragraph into the article describes Snowden's requirement to use PGP while contacting Greenwald at the least (although it's a tertiary source and very sparse in technical details).
It's not enough to merely tell people "install PGP." Snowden presumably went into meticulous detail about precisely how to be completely secure. Even something small like "here is the exact exe installer to download" is probably significant, because that would mean that specific installer is clean and free of NSA tampering.
As tptacek and others frequently say: "Crypto is hard to do right".
There is sufficient encryption technology to evade the NSA (at this point in time) but the social-engineering aspects and difficulty confirming a bug-free, secure implementation present the most issues.
[1] http://www.huffingtonpost.com/2013/06/10/edward-snowden-glen...
I wish Greenwald would publish that step-by-step email. It's probably one of the most valuable HOWTOs ever written, because only Snowden (and his colleagues) know for a fact what steps are NSA-proof.
Glenn Greenwald is really not very technical at all, so I imagine it was really about getting GPG installed and an appropriate plug-in for his mail client. I actually bet most of the e-mail was spent explaining why he needed to do this, because even now people are still reluctant to use GPG. There's been no explosion in it's use. People are still lazy.
But this is just the logical conclusion if you believe: a) Crypto math works b) The NSA has zero-day exploits for every system.
Recall, for example, Glenn Greenwald's admission that he "almost lost one of the biggest leaks in national-security history" because Snowden initially insisted on communicating with strong crypto and Greenwald didn't want to be bothered to install it.
What exactly did Snowden insist Greenwald do, precisely? Whatever Snowden insisted on, it's guaranteed to be an NSA-proof method of communication. So it seems like it's an essential first step to figure out the details and train people to use it habitually.