You need to remember that when those ads aired Windows still ran everything as root and had genuine "viruses", not trojans or other malware. But a virus as in the definition of the term. That Macs and Linux had a sane security model really did make them essentially "immune to viruses" in a very real way.

Today, a genuine virus is a rare thing, most malware are trojans. But back then viruses were the dominant form of malware. It's quite unfair to criticize people claiming Macs don't get "viruses" by changing the context on them to include all malware. That's not the claim people made.