I believe any software that, once installed on a system, gives someone else remote access to control that system is "a backdoor". So the malware itself is "the backdoor", it's not a case of "package X has a backdoor that was exploited".

Not all malware acts like a backdoor: some malware exfiltrates data, some seeks to destroy the system, some encrypts data to hold it hostage, some performs attacks on other systems using your CPU/IP/memory, etc. The malware they are describing here does act like a backdoor though, and doesn't seem to have other malicious behavior.