The executable appears to be packed using vanilla UPX. Decompressing it and grepping the symbol table reveals the presence /lib/libc.so.6, /lib/libm.so.6 and /lib/libpthread.so.0.

By method of exclusion, it cannot be musl because musl unifies everything into a single /lib/libc.so.

It most likely is not uClibc because uClibc is typically symlinked to /lib/libc.so.0 from /lib/libuClibc.

Considering that the soname of /lib/libc.so.6 has been reserved by glibc since the 2.x days (to differentiate it from the Linux libc days of libc4 and libc5), it is most likely indeed glibc in use.

It's obvious this program isn't making use of NSS or anything like that, so it's doable to use glibc for static linking, if frustrating.