I haven't played at Duelbits since last few months, still I received the free bet. So, it isnt related to the activity or inactivity of a user. Maybe they have sent this promotional message to some selected users.
It is same like their other promotions where they send these codes or freebets to some random users despite the fact they are active or inactive for months on duelbits.They have some promotion for inactive players in past I think to attract them to bet again but these are totally random so lucky are those who get it and turn it into profits.