If you read the various news reports closely, you'll see one vector was the 0-day IE vulnerability, another was malicious payloads in PDFs.

The headlines haven't helped.

http://www.computerworld.com/s/article/9144844/Hackers_used_...

"Hackers exploited an unpatched vulnerability in Microsoft's Internet Explorer (IE) browser to break into some of the firms targeted in a widespread attack that compromised Google's and Adobe's corporate networks"

"Google and Adobe, the only two companies to have stepped forward thus far to acknowledge the attacks, were hacked using malicious PDF files that exploited a zero-day vulnerability in Adobe's popular Reader software"