Many of Apple's flaws are not in its operating system, Mac OS X, but rather in software like Safari, QuickTime, and iTunes.
"...a growing trend in the world of security flaws: the role of third-party software. Many of Apple's flaws are not in its operating system, Mac OS X, but rather in software like Safari, QuickTime, and iTunes." How are any of those third-party? They are all sold by Apple. Just because they also happen to run in Windows doesn't make them third-party.
What Secunia did study was described as "In the first part of the report we look at the global picture covering all vulnerabilities in all products, followed by the analysis of vulnerabilities affecting the products and the operating system found on typical end-users PCs." And all their graphs are limited to XP, Vista and 7. So...probably limited to Windows. And they also define 3rd-party software for you as non-Microsoft vendors in the sidebar of page 9.
So yeah, everything by Apple is 3rd party software in the context of the study cited.