You're talking about the Insurance Files, right? Why would Assange give a toss if the government decrypted that? Its their stuff in the first place. Also, the Insurance Files were an exception to the way Wikileaks usually did business, not a demonstration of their typical use of encryption.

What use would a one-time-pad be with the only key around his neck when he got arrested? He needed a dead-mans-handle type arrangement.

There are lots of things that can be claimed about Assange, but claiming that he doesn't know encryption is not one of them.

ref: http://en.wikipedia.org/wiki/WikiLeaks#Insurance_files