Reading the posts from the linked topic, it seems to me that the forum has some prerequisites that must be met in order for the activity to continue after it is frozen. It is obvious that a member cannot continue to progress when it comes to activity points if he does not meet some other conditions, which is good in a way because it requires a lot more time and effort.
Maybe it's a little strange that we don't know the exact formula, but still the next rank that someone can reach after Legendary is Mythical, so it is logical that the progress towards that rank will be somewhat mystical
The exact formula is to reach a certain number of activity, in this case 2,000, and the time that the user has been connected to the forum. This formula is different for each range, and the waiting time until the activity is counted again depends a lot on each user. I seem to remember (I'll try to find the post) that my activity counter also stopped when it hit 2,000, but it only stopped for 48 hours or even less if I remember correctly.
As you say (I hope many people read this thread to understand it better) the important thing is to be active and participate in the forum, this way the activity will continue to increase and almost without realizing it our account will rise in rank without long waiting times. Spending time thinking that our activity is not counted will only distract us from the main objective, be active in the forum.