It had long been claimed by Apple that OSX was not "susceptible to the thousands of viruses plaguing Windows-based computers" and "doesn't get PC viruses" -- which is technically true. Viruses that run on Windows will typically not run on OSX.
You need separate viruses for OSX because it uses a completely different standard library, support APIs, and even executable format. Software build for Windows won't run on a Mac, and that includes viruses.
Note the nowhere in this claim is there the assertion that "there are no viruses for OSX"; such a position would be simple to prove false.
Why they now changed their position on the (trivially true) assertion regarding Windows viruses; most likely this is because by making that assertion, they're drawing attention to the now growing list of OSX viruses as reporters snatch headlines by pointing out the hypocracy.
By instead being silent on the issue, the could hope to continue to capitalize on the general perception of safety without actually claiming it to be true.