Horrible site, can't even cut 'n paste quotes from it...
Anyway, the writer seems to confuse risk and security. There's a statement that the security of an operating system depends on the size of the install base. This would be incorrect, the security of a system doesn't change, the amount of risk does. Because you're a bigger target you are more likely to be attacked. But this doesn't change the inherent security of a single system. You just get attacked more.
Suppose 10% of attacks are successful, regardless of the OS. If you have 10 machines with OS Y (purposefully not using X here) and 100 machine with OS Z. Only one machine with Y is infected, compared to 10 on Z. Looking only at those numbers you would, incorrectly, deduce OS Z is less secure than OS Y because there are more infected machines with OS Z.