That seems like a standard that is not really applied to any other post on HN. Why should it apply in this case?

Most other posts on HN don't involve such degree of mudslinging. When they do, I think you'll find similar questions being asked.

I don't really think is is mudslinging. He purposely left out the company's name, and YC as a whole won't be hurt by this - there have been enough YC successes that I'm sure everyone here would consider a YC startup to be better than the average startup.

This is just one engineer warning people that you should still take a solid look at a company before you hop on board, even if it is a YC startup.

How is he supposed to tell us the "other side of the story"?

Are you somehow involved in this? Do you recognise the situation?

Nope.