each member here on this forum have different opinions in regards of what is considered burst posting. Therefore I conclude that it had became an unwritten rule here, correct me if I'm wrong.
It's not an unwritten rule, but just a subjective matter.
An unwritten rule is like how someone want to send his sMerit, you can give 50 merits in one post by satoshi and no one will care about it. But if you give 50 merits in one post by brand new newbie, someone will create a new thread in Reputation.
I have remembered back then whenever there's a topic that I wanted to post/reply I'll write a draft ready to be posted and just wait for the time to hit 10 mins or so just because I don't want to consecutively post replies. Sometimes I would even open multiple tabs and reply at the same time but will just wait after few minutes before clicking post. Is what I did back then forbidden?
It's fine, there's no rule you're broke here. I will say that's a trick how you really want to give a gap time from one to another, there's no trace you leave if you use this.
While some regular members may think a 20 minute gap between posts is fine, campaign managers are more concerned in the visibility of the signature on different hours throughout the day.
If the user only post in the same thread, it's make sense the posts should have a gap time, but if the poster is create post in different thread/section, I don't see any difference why it's need a gap time.