They do however often ask friends and family to upvote and leave comments. Like "we have been very happy using XYZ" which is against HN rules but not that strictly enforced. I feel like it is extra-lax towards YC companies but maybe I'm imagining things.

On the contrary, we particularly tell YC founders not to do this—mostly privately, but there's a public version of it at https://news.ycombinator.com/yli.html (scroll down to "Comments" and see the part in bold. That's me trying to scare them.)

We do tend to be more lenient when there's no evidence of organized manipulation, just friends/fans/users trying to be helpful and not realizing that it's actually unhelpful. What dedicated HN users tend not to realize is that such casual commenters usually have no idea how HN is supposed to work.

But this leniency isn't YC-specific. We're actually less lax when it comes to YC startups, for several reasons.

Okay I have heard different things from some friends that are YC founders and saw them do it in practice (e.g. posting about their HN post on LinkedIn).

I'm not going to out people here but maybe it helps you to know that not everyone plays by the rules. Tbf I also understand that this is just really hard to enforce.

This is a great way to get your posts buried. We have to warn well-wishers not to do this (autonomously) before we post anything major, from bitter experience.