It probably uses stringification under the hood, but then relies on constexpr to "canonicalize" the name (removing everything before the dot, template parameters etc).
It also has to do work to extract the function name from the char arrays/C strings that those macros expand to.
Reading answers to https://stackoverflow.com/questions/4384765/whats-the-differ..., it seems it would have been easier to use the __FUNCTION__ in gcc, but I’m not familiar with that (or the other predefined values) at all, so who knows?
It's actually a pretty clever way to automagically deduce not only names of variables but also of enums :). Too bad it's most likely not maintainable in the long run as compilers probably change what their __PRETTY_FUNCTION__ (or equivalent) macros expand to in new versions.