When a topic is created in Discussion, it creates an activity saying that a comment was added to the forum (not the topic). Additionally, all comment activities are against the entire forum, not the individual topics.
There should be a specific "topic created" activity. It might also make sense to suppress the "added a comment" for the initial post on a topic, since you can't create a topic without creating an initial post, even if you leave it blank.
For active forums with lots of topics, having all comment activities listed against the entire forum will get confusing and useless very quickly. The comments are on a specific topic and should be associated with that topic. It would make sense for an aggregator to show "X posts in forum Y" for very active forums / users, but the activities themselves should be against the most specific container.
Replies should be presented as such as well, though they should probably still target the topic.
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