I agree with you there. There is much more money to be made smashing windows machines. It is also easier to do for the script kiddies who are just in it for malicious joy. I say easier because there are more tools available, more tutorials, more communities, more help for someone just starting out. If you want to go breaking a Mac, you are much more on your own. For windows, you have a whole back catalog of past vulnerabilities which have been detailed in full, often with openly available proof of concept code lying around. Also there is the fact that even a patched vulnerability can still be exploited since with the number of windows machines in the wild, you'll still find plenty of people without the patch.
However, my original point was that the article is correct. Eventually as Mac gains more market share there will be more malicious software created to exploit it. However the tipping point won't come when Mac reaches some magic home-use market share percentage, it will entirely depend on Mac's market share in business, since that is where the money and incentive is. The wave of malicious software won't happen on Mac until the serious bad guys who are in it for the money start to target it. After all, script kiddies by definition don't create their own tools or methods, but they do statistically account for the largest percentage of security risks. I'm not sure what market share Mac has in business and server settings, but I'm sure it is no where near 9%.