To quote the review (which I've read more than the source):

  The Zone Memory Allocator(common.c: Z_Malloc, Z_Free,
  Z_TagMalloc , Z_FreeTags) is still here in Quake2 but
  it is pretty much useless:
  Tagging is never used and allocation/deallocation is
  build on top of malloc and free (I have no idea why id 
  software decided to trust the C Standard Library at this
  point).
  The overflow detector (using the Z_MAGIC contant) is
  never used either.

So, unless I totally misunderstand things, it is half C stdlib, half Hunk allocator?