Wikileaks got its dirt from Russian hackers. They did not do the hacks themselves.
The same Russian hackers who actually hacked both the DNC and RNC, but notably (and impactfully) did not release the RNC data.
https://www.thedailybeast.com/cheats/2016/12/10/report-russi...
What are you talking about? There's no source that Assange or WL had any information on Trump, which they held back.
Your cited article only writes about the twitter DMs, in which the WL account asked Trump Jr. to leak his fathers tax returns. It doesn't mean they ever posessed them.