Pretty much the same thing that would be wrong with asking a javascript dev to explain grunt.

What if that C++ dev primarily uses visual C++ and Visual studios projects? What if the C++ dev primarily used Cmake, ninja, or scion (or any of the other thousands of C++ build tools).

What if that C++ dev never really needed to start a project from scratch?

The issue is testing someone on minutia doesn't tell you anything about how capable they are. In particular, testing them on minutia that they very likely rarely interact with is beyond pointless.