It isn't hard to deny that general impression you're reporting, because it isn't true. But I don't see any way to refute such a sweeping claim convincingly. As far as I can tell it exists only on the level of rumor and is unanswerable. But I'm happy to reply about specific cases.
In the Drchrono case, we got an email from the founder asking to post a response after the commenting window had closed. I agreed, but not because this was a YC startup, but rather because we would do this for any startup in this situation, and have indeed done so for at least one non-YC startup I can remember. Before I agreed to the Drchrono founder's request, I told him I needed to make sure that we would do it for any startup in that situation, and I thought long and hard before concluding that was true. It was by far the most important factor in that decision.