This is off topic, but I don't think the problem is just that the Twitter is terrible.

If you want to link to a thread, you should click on the first tweet of that thread and copy its url. It's easy to see the rest of the thread: just scroll down.

If you want to link to one tweet in a thread you should click on that tweet and copy its url.

The person who posted the link to HN did the latter, while they should have done the former.

Twitter could make it easier to see that a tweet is part of a thread though.