Earlier quoted context omitted.
Whoa, no kidding. The majority of people are not going to stop on those two letters and consciously differentiate "virus" from "PC virus."
"A Mac isn’t susceptible to the thousands of viruses plaguing Windows-based computers." Pretty clear.
1. "A Mac is susceptible to viruses. But it is not susceptible to viruses plaguing Windows-based computers."
2. "A Mac isn’t susceptible to viruses, whereas a Windows-based computer is susceptible to thousands of viruses."
The fact that you still get viruses, but they just don't happen to be the same viruses, isn't worth stating. So as a customer, it is very unlikely that I would infer the former (Meaning 1) from the statement. Yet it is what is meant.
I would personally call this misleading (and dangerously close to lying).