To "fall foul", i.e. be required to add highly effective age assurance, there's a number of tests you have to pass

One of the tests is:

> Are there a significant number of children using the service or is the service likely to attract a significant number of children.

I'd guess that HN would be in scope for the act overall - they provide user-to-user functionality and have a lot of users in the UK. Either they answer no to the questions above, or they answer yes and should have performed a risk assessment where they look at things like what kind of content is allowed, how the site is moderated, how do users contact each other etc etc.