Exactly...just think how many people have malware and don’t even know it because they thought their Mac was malware proof.
I think it likely that the sample would contain a higher proportion (how high is impossible to say) of infected computers than those not downloading anti-virus software.
Most malware these days is based on social engineering; the operating system is not the primary vulnerability that it once was. As long as clueless users can be tricked into installing malware, it will continue to spread.