Based on the user rank in the BleepingComputer article's screenshot ("kilobyte") it looks forum.exploit.in. Anyone can buy a membership for ~$100 in BTC. I've read some of these places and honestly it's pretty boring. It's basically just a marketplace for software to steal money from people in first world countries (stealers, n-day exploits, crypters, AV scanning services, phishing services) and a bunch of supporting services like residential proxies, hacked RDPs, SSN lookups, and "drop" bank/crypto exchange accounts.

There are others that are public: antichat, wwh-club, bhf. I'm not on it but I think the most "elite" forum is called Mazafaka and requires a substantial deposit and a recommendation from existing members.

Exploit.in is more technical than most of these forums but a lot of these people are not really very technically skilled they just have a very good understanding of how anti fraud systems work and how to circumvent them.