Greenwald spoke to the media, in Portuguese, at the Brazilian airport where he met Miranda upon his return. Here is the full quote from Reuters' coverage:
I will be far more aggressive in my reporting from now. I am going to publish many more documents. I am going to publish things on England too. I have many documents on England's spy system. I think they will be sorry for what they did. [...] They wanted to intimidate our journalism, to show that they have power and will not remain passive but will attack us more intensely if we continue publishing their secrets.
The Reuters report focused on Greenwald's "they will be sorry" comment and implied that Greenwald would be publishing more documents in response to the government's decision to detain his partner. Greenwald took issue with the framing, saying the Reuters report neglected to include key context, including the questions that prompted his comments. He tweeted out this paraphrase of his conversation with the press:
Q: Will the UK's detention of your partner deter your future reporting?
A: Absolutely not. If anything, it will do the opposite. It will embolden me: I have many more documents to report on, including ones about the UK, where I'll now focus more. I will be more aggressive, not less, in reporting.
Q: What effect do you think they'll be of the UK's detention of your partner?
A: When they do things like this, they show the world their real character. It'll backfire. I think they'll come to regret it.
But other news organizations had already followed Reuters' lead in sticking to the narrative that Greenwald was threatening the British government over the incident. The Huffington Post's headline blared: "Greenwald vows vengeance," while many others hooked on to the alleged "sorry" comment.
It's probably not surprising that the "sorry" angle took off in the media -- it's an attention-grabber, and it's easy to sensationalize outside the alleged full context. But even if Greenwald hadn't directly disputed the Reuters presentation of his comments, the Huffington Post's vengeance interpretation is a little far-fetched.
Greenwald's point seems to have been that he was determined not to be scared off by intimidation. Greenwald and the Guardian have already been publishing documents outlining surveillance programs in Britain, and Greenwald has long declared his intention to continue publishing documents. By doing so, Greenwald isn't taking "vengeance." He's just doing his job.