At the time of the article (~5 months old), the folks at Symantec said the number of Jnanabot infections so far is “measured in the thousands.”. What you are looking at is the state of trojan 5 months later, after it has been detected and removed from many systems.
Whatever the peak numbers were, I stand by my hypothesis that the Macs were running Boot Camp. The technical info on Symantec’s site (http://www.symantec.com/security_response/writeup.jsp?docid=2010-102616-4246-99&tabid=2) lists only Windows as vulnerable, and describes the mechanism of placing entries in the Registry and downloading .exe files, neither of which affects Macs.
The pie chart was based on “artifacts of the file system.” A Mac running Boot Camp does mount the Mac OS volume, so that file system would be visible. The blog post does not describe how, or whether, they account for systems with more than one OS present.