comments are threaded. just just need to use the correct reply button. to start a thread you reply to the individual post (the reply button within the message you want to reply to). to reply to the whole topic, ie, reply to the original post, use the blue reply button at the bottom of the list of posts.
the interface updates and you can then click to see threaded replies to a comment. and also floating comments show an “in reply to” icon which will expand the original comment.