Shivaun Albright, HP's chief technologist of print security, said at the time:

"A researcher found a vulnerability over the serial interface between the cartridge and the printer. Essentially, they found a buffer overflow. That’s where you have got an interface that you may not have tested or validated well enough, and the hacker was able to overflow into memory beyond the bounds of that particular buffer. And that gives them the ability to inject code into the device."

Albright added that the malware “remained on the printer in memory” after the cartridge was removed.

So HP had a vulnerability in their printer's firmware that allowed arbitrary cartridge code to become executable, and they're trying to spin this so it doesn't sound like their printers are at fault. Still sounds like a them problem.