Probably easier in NA due to weekly+daily+other arp extras+arp boosts. But yeah 400 is A LOT.
Outside NA I don't think possible to earn so much arp in a week unless there is cheating involved.
It is'nt possible in NA too unless your the first one to solve the weekly quests with booster maybe you could get around 200 ARP. And the fact that there are multiple users with over hundreds of ARP at the end of the week seems very sus to me or they have some miracle ARP producing machine. Right now I am contemplating what I would do if I could get my hands on some miracle ARP producing machine..
Make a banger of a news article plus do all the quests and activate an ARP booster.
Looking at their profile none of these individuals stood out with any user created content. But before we grab our pitchforks the admins will give us a plausible explanation wouldn't they?