That said, he appears to be dealing in absolutes too much. If you care about performance (and let's face it, if you're using C you do, otherwise you probably shouldn't be using C) then sometimes you can't handle as much error checking or error correcting as you'd like.
In games (where most of my experience is), it's common to have functions that are 'unsafe' by his definition, but that are hidden in compilation units and not exposed in the header, so that the programmer can control exactly where they're called from. If you have a limited number of 'gatekeeper' interface functions that are actually called from outside the module, and these either check/sanitise/assert on their inputs, then the internal private functions can safely assume that they have valid input and just run as quickly and as simply as possibly.