Players can be shown inactive even if they are active because they stay on the same page without changing it or refreshing the game and if I want to attack them after they get offline for 15-20 mins and they are on a page for that long I will hit them when they are online.

I tested this with someone and this appeard after 20 minutes after being on a page 

After this I dm'ed the player and he replied me back even when he was idle, after he refreshed the game it appeard online.