Obviously, he doesn't have to say anything positive about Trump at all to support him, and in fact the kind of support he's offering (a drip-drip feed of the products of Russian hacking calculated to create an "October Surprise") is far more valuable than any verbal support he could give Trump directly.
I mean here to be dismissive without being too disrespectful. But I'll be candid: people who say Assange isn't supporting Trump sound pretty silly. Like, the claim says more about them than it does about the election.