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.
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.