That is the expectation but no way to enforce it of course.
What happens a lot, at least for me, is that people will start reading the comments to see if they want to bother reading the link. Then they might start commenting on what's already been said. It's easy to slip into that pattern.
Though you also frequently see top-level comments that appear to be based on the headline alone.
Most people on Hacker News don't bother to read the linked article and either comment based on their impression of the title or whatever random thing happens to be on their mind at the time. Most people who do bother to read the linked article stop as soon as they encounter javascript or formatting or too much whitespace or a minor logical, spelling or grammatical error and then that will likely become the subject of the entire thread.
The number of people who actually read the entire article and then attempt to comment in good faith are few and far between.
