Bespoke malware is not a "backdoor". By that loose definition, every RAT out there is. They're so common they don't get fancy names anymore. This is clickbait.

The xz backdoor was such because it was stealthily slipped in with the vendor's original software, without their knowledge, and no further attack or action by the user was required to install it.

The Juniper backdoor a few years ago qualified per the same criteria.