This is also why functions can work on a null object and not crash, as long as they do not access instance data. Though at that stage they should be static or external.
I imagine the author was an old-school C programmer who understood well how the C++ object veneer maps down to equivalent C code / assembly. From that perspective, dealing with null `this` pointers is just another way to get the compiler to spit out the assembly instructions you want, and maybe enables some elegant patterns in client code... but it's exactly the kind of thing that makes newcomers hate C++.
(&x)->vtbl[N](&x, y)