On some platforms, malloc(3) and free(3) are implemented in terms of mmap(2) and munmap(2). Which means, if you go beyond your buffer space, you'll end up in an unmapped page and cause a SIGSEGV or end up in NULL data.

Since they used the freelist/malloc wrappers, they avoided this trap.

Plus, IMO, if you're implementing a freelist, you're implementing 80% of what a malloc is anyways. It's _mostly_ freelist handling, the only way to add memory to a process aside from mmap(2) is setbrk(2). There's no way to give memory back, so they only thing left for a malloc implementation to do is create a freelist to reissue memory previously freed.

You can say you're still backed by malloc(3) if you want, but you're only getting the setbrk(2) handling and throwing away everything else it's trying to provide you.