The conclusion that Russia wanted Trump to win comes from the U.S. intelligence community’s assessment discussed in the Washington Post piece my article is responding to. Though I mentioned some parts of the Post’s report, I recommend reading the whole thing. There’s a link at the top of my article. (And here if you missed it).

Note: saying that Russia wanted Trump to win is not the same thing as saying Trump colluded with Russia. I’m not aware of any concrete evidence that he did, and, as you note, Comey said the president was not under investigation for it.

The assertion that the hackers sat on whatever else they found is based on the numerous reports they stole information beyond what they released. If they didn’t release all of it, that means they’re sitting on some of it.