Well ... except by definition bitcoin is hashing at that rate AND computers are being used for other tasks, so they can't be using 100%. There are also billions of embedded systems that have computing power but can't be used for bitcoin mining. What about CPUs in phones and tablets?

If you define "world's computers" as "desktop PCs" and you exclude GPUs (which my son seems to use to heat his bedroom), then maybe this statement makes some sense ... but it's really a pretty weak milestone.