> But those are technically the OS system calls, not libc.
They're libc calls, libSystem is the union of a bunch of libraries including libc. In fact, these unioned libraries are just symlinks to libSystem:
libc.dylib -> libSystem.dylib
libdbm.dylib -> libSystem.dylib
libdl.dylib -> libSystem.dylib
libgcc_s.1.dylib -> libSystem.B.dylib
libinfo.dylib -> libSystem.dylib
libm.dylib -> libSystem.dylib
libmx.dylib -> libSystem.dylib
libpoll.dylib -> libSystem.dylib
libproc.dylib -> libSystem.dylib
libpthread.dylib -> libSystem.dylib
librpcsvc.dylib -> libSystem.dylib