But yes, you're right, they could have developed Glib in either of those languages even if STL and *step didn't exist yet. I also found this e-mail[1] from Stallman that supports your claim. Do you know of more documentation? I'm interested in this history.
Here is one of the original versions of the GNU Coding Standards from 1994,
https://www.schwedler.com/old/HTML/standards_7.html#SEC12
> Using a language other than C is like using a non-standard feature: it will cause trouble for users. Even if GCC supports the other language, users may find it inconvenient to have to install the compiler for that other language in order to build your program. So please write in C.
https://web.archive.org/web/20041029162424/https://www.gnu.o...
As you can see, it basically consisted in "use C, unless you _really_ have too". It was also well known that C++ was heavily discouraged by Richard Stallman, so picking it could well end up in your project having a hard time being adopted by GNU.
The guidelines were relaxed afterwards, especially since GCC came to rely on C++ and every relevant C compiler was then written in C++, making the whole "we don't want to force people to install g++" even more nonsensical. In 2004 C++ was already way too ubiquitous to not have g++ installed.
I think that Glib was born from the same mentality which considered C as the perfect language, with the end goal of getting the same functionalities of modern languages without leaving it.