He hasn't strongly denied it. He said in 2011 that he would sue them for reporting the quote. He hasn't. Anyone can say "it's not true" on twitter. I don't call that a strong denial.
In any case, the names of informants were published. He did it. That's all you need to know about the man.
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.
Just bear in mind that libel is a seriously expensive game in the UK (£100,000+), and perhaps might not be your highest priority when you're fighting an extradition attempt.
In any case, the names of informants were published. He did it. That's all you need to know about the man.