I don't know if Mac users pirate less than Windows users, but I strongly doubt it.
However, many indie & shareware developers who produce products for Windows -and- OS X say that usually one third to one half of their income comes from Mac users, which indicates that Mac users, given their much smaller market share, tend to purchase more than Windows users.
One reason might be that you actually need to purchase more software for OS X than you need to purchase for Windows, because there is more Freeware available for Windows than there is for OS X.
I would also support the argument that the average Mac user has a thicker wallet than the average Windows user - after all, a Mac user is willing to pay a premium for Apple's designer hardware and software, indicating that he has more money at his disposal. (There was only a very small time window when the brand new Intel Macs could bet most of the PC competition over the price. That window has closed quite a while ago, and now we're back to "normal" Apple business.)
One the other hand, the premium priced hardware could also be a reason to pirate more software - especially when the respective user is on a budget and more or less has spent his budget on the hardware already. Then he's forced to pirate software if he wants to have a useful system. (One certainly does not need a Mac when Open Source software perfectly satisfies one's software needs, so pointing at Open Source software is not really a valid argument here.)
In short: Apple users might spend more money on software and content, but that does not automatically mean that they pirate less than others.
The article says that Mac users report that they are more likely to buy music. I expect that's due to the ease iTunes provides, if true.
Only that iTunes is also available for Windows and most iPod owners are Windows users.