The reason we make web pages is so they can be accessed, not so they fit some dubious image of technical purity. What they did seems fine to me.
Web standards are part of the reason why your web page can be accessed.
The HTML5 spec is very explicit about which tags are auto-closed, and it is neither a parse nor validation error if those tags are omitted.
I wish HN prevented new accounts from commenting for one day unless verified by a mod.
I think the community moderation has done its job here, and the negatives from preventing new accounts from commenting wouldn't be worth the advantage.