The OA uses the term "Linux backdoor," but then goes on to describe it as a add-in kernel module. It's not a backdoor, but rather a rogue kernel module someone has written. The module in question, ipt_ip_udp, isn't part of the Linux kernel. It's merely a module some black hat wrote to provide remote access to an already compromised system. This is just FUD and self-promotion by NCC Group to make what they found sound much more important than it really was, no doubt to increase their client base. What crap.
To sum up, it isn't a Linux back door and it isn't a vulnerability in the Linux kernel source code. It's merely a rootkit.