HN is absolutely social media and it does have some of the dark patterns that plague other platforms. They're just more reigned in. A change in moderation policy or new moderators could destroy this site in a week.
I personally don't think kids need to be banned from participating here. However, the law is often a blunt instrument and it's probably better to get kids off of Facebook and HN if distinctions cannot be made.
I'll try to convert it into a metric: measure the number of involuntary users via the comments saying "I hate this website". You rarely see people here saying HN is bad to the point of being a net negative on them, for example, but this is true of all normie sites, including reddit.
No this isn't true at all, it absolutely does matter legally. Look at Australia's underage social media ban. Twitter was forced to ban children, but Bluesky was not despite being the platforms being effectively the same. Roblox and Discord, no bans despite being an extremely common place for young people to socialize.