Hello!
I think you doubted about this requirement because on the application, it said in brackets '(We can tell if you have been AFK)'. First off, as everyone said it is being on HHGN for at least 1 month, not time wise!
It said in the application that 'we can tell if you have been AFK' because a player can AFK everyday to show that he or she is active. For example, if a player has been on HHGN for a year, but has only joined twice since he/she joined, then this will automatically deny his application.
It places great emphasis on the activity and time spent on the server, not only for the sake of being on the server for atleast a month.
Hope this made sense.