a frequently updated Android distribution (Nexus or CyanogenMod) might in fact be more secure than iOS, where you are forced to be vulnerable to Apple's Webkit engine.

I took that to mean that Apple devices are more vulnerable because they are infrequently updated, as compared to Android. Google and its partners do release fairly frequent (every 3-4 months) dot releases of webkit, Chrome, and the entire OS, to add features and address vulnerabilities.

By contrast, Apple's release schedule is rather monolithic and their superior security is based on a more tightly controlled platform.