I concur, there was a few problems that might come up on various platforms like Windows not implementing C11 standard threads and other stuff, you would instead use TinyCThread library that works like a polyfill.

All problems and challenges are workable, if the problem with Debian is out of date library, you could set up CI/CD for release build that rebuild your software when update occurs and static link the updated dependencies.

Back to your point, if they didn't design their code and architecture to be multiplatform like in C, they need to re-evaluate their design decisions.