Forums update threads by bumping, so threads from ten years ago can still be on the front page as long as they are active.
The term "link aggregator" was made to differentiate websites that are designed for threads to rapidly decay and be replaced by a constant flow of new content. If you tried to federate lemmy with a forum it wouldn't really work.
Maybe there's another term that could be used, but there needs to be a way to differentiate the two styles.