Namely, from the follow-up article
"Mr. Miranda was in Berlin to deliver documents related to Mr. Greenwald’s investigation into government surveillance to Ms. Poitras, Mr. Greenwald said. Ms. Poitras, in turn, gave Mr. Miranda different documents to pass to Mr. Greenwald. Those documents, which were stored on encrypted thumb drives, were confiscated by airport security, Mr. Greenwald said. All of the documents came from the trove of materials provided to the two journalists by Mr. Snowden."
In the helpful clarification from Wonkette
, "he was actively participating in transporting secret documents that were stolen, and which it is illegal for him to possess." On a trip paid for by The Guardian.
So, maybe not quite as innocent a bystander as he initally makes it seem. But that was probably the point, and now British politicians are getting hammered for the abuse of power he baited them into. Well played!