She does what she can do and what she is allowed to do by her bosses. I wouldn't say that she's a bad community manager man.
While I believe your correct, there is no 'hard evidence' regarding your statement. We only assume, and logically so, that Relic's NDA also applies to her work. But we haven't read her employment contract, and 90% of us haven't read Relic's NDA.
In any case, I'm sure advertising streamers on facebook isn't part of NDA.
On a slight tangent here, Riot Games also has quite an NDA policy. Among other things that really annoyed the players why Riot couldn't give constant status updates on champion reworks, when NA-East Coast servers were coming, what 'Riot Supremacy' was, 'Magma Chamber Where', etc. The company also explicitly made a statement on this, and I paraphrase here, 'We don't want to say everything because we may not meet our promises and other companies may steal from us. Look at Heroes of Newerth and their 'casual mode' where you can't deny creeps. Guess where they got that idea from?' But that didn't, and still doesn't, stop 'Rioters' from actively engaging in Community discussions. Spend 5 minutes on Riot Forums, use the 'red tracker' (red = riot employee) and see how many posts they have made in the past 24 hours.
There was a thread a while back where Rioter gave their opinions on the 2012 US presidential elections.
While not all of these Rioters are community managers, it doesn't change the fact that Cynthia has made next to 0 posts (She probably has less than 10 total posts, excluding changelogs). Some of these people, like Ironstylus (artist) at Riot post more than I do.
I'm not accusing her of intentionally being bad at her job, and I don't believe that that is the case. I'm simply stating that nothing explicitly rules out that possibility.