Indeed the NPCC 39 bus system is neither a distribution system, nor of a radial structure. It represents a power transmission network with centralised generators connected to it. I used this system during my PhD study at year 2002 for electromechanical transient studies. Below you will find the thesis and a recent book (year 2015) that presents also the majority of that thesis content.

https://www.researchgate.net/publication/307585561_Electromechanical_System_Equivalent_and_Dynamic_Load_Aggregation_for_Transient_Stability_of_Electric_Power_System

https://www.researchgate.net/publication/307584190_Dynamic_Security_of_Interconnected_Electric_Power_Systems_-_Volume_1

So can I treat the same system as an active distribution system?

There are many standard distribution test systems that can be transformed into active distribution systems. Therefore, I cannot find a reasonable use of a standard test power transmission network as an active distribution system! Do not you agree?

yes you are right! Actually the paper confused me because it says it is a radial distribution system.

Based on my experience as a reviewer and also as a supervisor, there are many authors and students that insist on using irrelevant keywords to their work. They are doing that for showing a fake importance to their work with the hope of tricking the supervisors, editors, and reviewers. What a shame!

Its an interconnected power system contains transmission and distribution network

They are the same and I think Sir Mohamed EL-Shimy clarifies your query, and for me, that's fine.

Than you Sir Umair Shahzad and kind regards.