This account is having some issue. How can he get 30 activity in less than 28 days ? As per his profile :Date Registered: January 16, 2018 , this is just 21 days since he created the account.
I think you still haven't understood how this activity works. This is practically possible. We can become a junior member based on the date of registration and that too within 28 days. This can be made true by potential activity. According to the activity change spreadsheet time mentioned here :
https://docs.google.com/spreadsheets/d/12saLhlUoqIdairxzuSPu6EYGrt7FN2lOstO1yDjCEbA He has registered his account luckily before the activity time completed which is on Jan 16. Hence due to this fact he has received 14 potential activities for this period and another 14 potential activities on Jan 17. Hence he has 28 potential activities and he can reach 28 activities within 2 weeks= 14 days and he reached 30 activities within 28 days as he has posted in third 2 week period. So only he has reached the Jr.member rank within 28 days.
Potential activity is nothing but number of 2 week period you have posted. If you post in three 2 week period you could become a Jr.member and that's what he has done.