I can't speak to the "important" in that claim, but the folks at ZH have friends and connections in the world of finance from whom they can get leaked communications and reports.

For example, this post on Trump's nomination chances [1] draws heavily upon charts produced by Goldman Sachs, presumably for their clients only.

[1] http://www.zerohedge.com/news/2016-03-20/can-trump-win-gener...

Edited to add: I also know this because a friend of mine had their report leaked and written about on ZH, leading to a bit of a storm in a teacup at their workplace.