Syscalls are provided by the operating system, it doesn't matter which C library you use. The C library usually just provides a nice API to call the syscall. And it it doesn't, you can still call it directly via syscall(1).
Yes but that's not what this table is describing. The table appears to be describing cosmopolitan libc's wrappers. Certainly most of these syscalls do not exist on Windows, so the "Windows" column couldn't possibly be describing raw syscalls.