Glibc is LGPL, which makes some exceptions for system libraries, which glibc would fall under. It's been a while since I've read it and IANAL, so I'm not sure what exactly would be allowed.
The bigger problem is that glibc cannot be statically linked properly. It dlopen's some libraries, like for NSS. Maybe games could avoid triggering those cases but musl is definitely better.
While syscalls themselves are backward-compatible (even for faulty behavior), I think I've read somewhere that some parts of DRI are not, but I've lost the source on that.
The bigger problem is that for games you will to interface with OpenGL/Vulkan and probalbly Pulse/ALSA and those are all shared libraries which you cannot use from a completely static executable.
The bigger problem is that glibc cannot be statically linked properly. It dlopen's some libraries, like for NSS. Maybe games could avoid triggering those cases but musl is definitely better.
While syscalls themselves are backward-compatible (even for faulty behavior), I think I've read somewhere that some parts of DRI are not, but I've lost the source on that.