I wonder why the implementation is all in a header.

This is a typical idiom frequently seen in modern C and C++ codes. As https://github.com/krychu/wfc#how-to-use-the-library says, you need to define a macro in a C file in order to "expand" the actual code there.

Having the code in the header gives a bit more flexibility considering the file layout. IMHO this is an awkward consequence of the missing de-facto-standard in C/C++ build systems.

How is it more flexible exactly?

I hate this idiom every time I come across it. I like to look at the header file to see the interface and important documentation, and this just obscures it. Depending on your compiler it can make debugging a huge pain as well.

It does seem that 'modern' idioms tend to just be doing the same thing in a more obfuscated manner. Just a .c/.h pair would have been easier to add to your source.