The link you provided refers to a New Statesman article about a debate held to discuss the effect of whistleblowing. I quote the relevant passage from the New Statesman article:
Murray didn't back down, moving on to make reference to the Guardian's claim that Assange "didn't care" about Afghan informants who were identified as a result of the release of the war logs.
Assange hit back: "Point of order! We are in the process of suing the Guardian . . ." After a bit of back and forth about libel laws – Assange said he has campaigned for their reform, but that people should have recourse when allegations are made against them – Murray drawled: "I think I'll take from that that Mr Assange thinks libel law is good when he's using it."
I cannot see where Assange said '"it's not true" on twitter', as you report, but in any case it seems clear that he didn't just make some off-hand remark on a social medium, as you seem to suggest.
In any case, I don't see why using twitter as a platform would reduce the validity of such a statement. There's a difference between tweeting to your followers you're going vegan and claiming you're suing the Guardian for libel.
>> In any case, the names of informants were published. He did it. That's all you need to know about the man.
To be frank, I don't much care about statements that start with "that's all you need to know". I usually prefer to chose for myself how much I need to know of anything.