HN is reflecting that in YC's movement.I think it's uncontroversial that there are likely a large number of HN members who have favorable opinions of Y Combinator. That said, I don't see evidence of Y Combinator exercising any kind of persuasion or manipulation of the members where one can reasonably say that HN members are following Y Combinator's lead. And there's enough vociferous, contentious, argumentative political discussion on HN that it's clear to me that HN doesn't represent a single bloc of anything other than all participating in the HN forums.
As for YC becoming the liberal version of the Koch brothers, I think it's hard to underestimate the difference in scale. In January 2015, the Koch brothers budge for the 2016 campaign was nearly $900 million.[0] Along with Rupert Murdoch, the Koch brothers largely bankrolled the Tea Party into national government[1] in 2010.
Another significant difference, in my opinion, is that the Koch brothers themselves are not very vocal or public. They operate largely behind the scenes. Y Combinator has been much more vocal about what they're doing.
And it bears keeping in mind that YC has kept Peter Thiel on as a part-time partner: if YC's motivations were purely political, I think it's fair to say they'd have cut ties with Thiel just as so many have been clamoring for them to do.
All said, I don't think the comparison has much merit.
[0]: https://www.nytimes.com/2015/01/27/us/politics/kochs-plan-to...
[1]: http://www.nytimes.com/2010/08/29/opinion/29rich.html