Other than #define'ing SO_REUSEPORT, libc has no role whatsoever in its behavior. You can see this yourself by grep'ing for SO_REUSEPORT in the glibc and musl source code. And both glibc and musl implement 1:1 threading, so it's the kernel making all the thread scheduling and inbound connection queueing decisions.

Your problem lay elsewhere, unless you were using a really old version of musl that lacked the SO_REUSEPORT definition.