But there was a time when activity = posts, I think in 2012 or so.
no
if you post only once you get 1 activity
if you post twice you get 2 activity
it's just that if you made at least 1 post you're considered active
and you can carry over the remaining 13 unused activity to the next 2 week period
so to gain those extra 13 activity you would need to make 27 posts instead of 14
Exactly.
So if you haven't made any post in the period, you "lose" 14 activity.
But if you have made just a post in the period, you still keep the ability to "claim" that 14 activity.