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.
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.