Not "closed loop" but "isolated" is what I think you meant. (Very different things.) But yes, if the virus were so written it could be conveyed by a memory card/stick.
Interesting that it appears designed to specifically attack Siemens SCADA systems. This was no accident.
I also read something earlier that the worm attacked PLC (Programmable Logic Controller) programs. Don't know if you're familiar with PLCs. They probably control 90% of automated systems, worldwide. Of course, Siemens is one of many manufacturers of PLCs.
Lots a speculation...