<…> Does it mean that any Jr, Member account will post 480 post(not activities) can directly become a Hero Member? Or did I misunderstood somewhere?
Hero Members require at least 480 of Activity and 500 Merits. Both requirements need to be met, and nowadays frankly, except for a very few people, the Activity requirement is met first and the Merit requirement is met way later. Meaning that, for the average Joe, Activity is totally secondary as the Merit requirement is the one that is hard to achieve.
Now strictly back to Activity, no, the 480 posts (nor Activity) do not get you to the Hero rank as mentioned above. That was before the Merit System, and under certain conditions: 480 posts, all posted in the same week, will not get you to the 480 Activity counter. You need to post 480, and have at least posted one of them in at least 35 distinct non-overlapping two week intervals.
So roughly, the shortest requirement would be to post (providing it is not deleted) every two weeks at least once for 35 weeks, and making the aggregate over that period for at least 480 non-deleted posts. That's just to meet the Activity requirement. The Merit requirement is another matter as we know.
See also (Activity based information):
https://bitcointalksearch.org/topic/the-definitive-explanation-of-how-activity-works-495948