Of course, sorry. I did not perceive it as such. I was just puzzled that the "30 posts" was wrong.
It's not "wrong", just incomplete. You need 30 activity, for which 30 posts are one of the requirements.
The full explanation would be 30 or more posts in at least 3 different activity periods.
Also remember that activity doesn't update instantly once you make a post, it can take some time until your activity catches up to the number it should be.
I should rather edit my past posts if I want to add something to this - right?
Yes, better that way. Quoting the whole posts and adding new lines with little to none content certainly isn't the way.