If HN was self-hostable software like Lemmy, would you host an instance?

Do you mean an instance that is tied to and moderated by HN but distributed to handle a bigger load, or a completely stand alone website where I must moderate it?

If distributed I might consider it. If it's just my site probably not. If it's my site I would have to pick topics that are not covered by HN and most of the interest here I have seen that is off topic is politics. That would mean I have to set up a site that can take a serious paddlin' which I have done and it's a lot of work. I'm lazy(er) these days.

I'm not sure I really understand the question -- it seems like it's just asking "would you ever host a discussion board". Am I understanding correctly?

If I were to entertain standing up a board like HN, I wouldn't use something like Lemmy to do it. I'm not sure what benefit I'd get from that. HN is pretty basic and, in my opinion, that's an important part of the appeal.

I don't see any reason why I would want to.

only if the server program can scale down to zero (aws lambda + dynamodb type of deployment)