what...?

   // NOTE that we use a while loop here deliberately.
   // inside the loop body we actually change placement_address
   // by calling kmalloc(). A while loop causes this to be
   // computed on-the-fly rather than once at the start.
   int i = 0;
   while (i < placement_address)
   {
       // Kernel code is readable but not writeable from userspace.
       alloc_frame( get_page(i, 1, kernel_directory), 0, 0);
       i += 0x1000;
   }

uh........

From quickly looking at the code, it seems get_page calls kmalloc_ap, which returns then increases placement_address.

The site in question is not very credible.
it's physical location
the physical addresses of it's page tables

Competent people don't usually confuse "its" and "it's".