It is a single-header library in terms of the format, yes. But it is also not designed to be a single-header library, unlike an STB-style library.

Nuklear is a full-sized library that can be compiled into and used as one file. However, RGFW and STB are designed to be lightweight and minimalistic. That's the distinctive part of single-header libraries, also known as STB-style libraries.

To be clear, I don't mean this to hate on Nuklear, it's a cool UI library, but its design doesn't match the single-header format.

What is this supposed to be telling me? This also looks like it was multiple files concatenated together because there are multiple different header guards throughout the file.