It's not at all clear. It's deliberately ambiguous. If they wanted it to be clear they would have said "Macs are only susceptible to a fraction of the number of viruses plaguing Windows-based computers."
Clear? One possible interpretation of just that statement alone could easily give the impression that viruses in general are therefore not an issue for Macs.
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).
Yes. They are stating that Macs are not susceptible to PC viruses. Yes, this is true. However this is not worth stating, so most people would infer that Macs cannot get any viruses from that sentence.
It may not be worth stating to people round here who are essentially experts in the field, but to the vast majority of everyday PC users it's not at all obvious and very much worth stating.