They should definitely be using the full width of screen for longer form "notes." I personally would also prefer a modal as opposed to a completely different page.
I find HN comments practically unreadable because of the lack of wrapping. The default render width of a tweet is perfectly reasonable. Maybe you could extend it out to the width of a substack or medium post.
But there's nothing natural about stretching a sentence out across 800 horizontal characters. It's just miserable on a good monitor.