It's stated several times that this is a cable specifical designed for penetration testing by a security researcher. So, yes, it's a feature.
The point of the article, and the company that scanned the cable, is that these kind of electronics can be surreptitiously built into cables for malicious purposes as well, and that a regular X-ray scan was not even able to detect the secondary IC. That makes them a potential threat in espionage and terrorist actions.
It's also one of the reasons why, at least when I was still working, computers in secure military and intelligence installations, as well as various corporate machines with access to restricted data, all had all their accessible USB ports physically disabled, and their IO devices permanently attached.
Securing against these sorts of threats has definitely become a lot harder over the past decade+, though.