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.
Believing that sort of dichotomy is "pretty silly" and says plenty about you.
Through I must say, it does making viewing news more interesting. Someone say a negative about a product, I wonder which competitor paid them? Someone released a security vulnerability, I wonder what competing product they are trying to sell.
It basically makes all negative statements into a conspiracy, which honestly is silly, but fun thought experiment.
Neither case really paints a good picture of Wikileaks as an impartial journalistic organization.
And the proof is...