No, there is a distinct difference... A Trojan relies on psychology to attack a computer, a virus replicates and installs itself on the target computers. Granny's machine will not install this on other computers without THAT user's intervention. There is no vector.
Without the "victim software package" the client computer will not connect to Granny's computer. It will ignore it and connect correctly to Apple. Just because Granny's compromised computer is on the same network does not cause other Mac's on the network to spontaneously also become infected. The only computer that will download malicious software will be Granny's. Trojan, not virus.
So as I said if I configure Granny's computer to act as the Malicious spoofing server...all Macs on her network that try to download a legit patch will get the trojan/virus.