They are implying that viruses are a severe problem on PCs that Mac users do not face. This was undoubtedly true when they were running those adverts.
They're saying PCs get viruses and Macs don't because of the way it's designed. Not that PCs get PC viruses and Macs get Mac viruses.
They are also implying that it's the sole reason for the lack of viruses, when it's mainly the lack of users. That would explain why it's not susceptible to every other type of software available on Windows.