The claim is one of those things that's kinda true, kinda not.
Snow Leopard is a 64-bit operating system. It is able to use 16 GB of RAM.
However......
By default, it boots the 32-bit kernel, which isn't capable of directly addressing 16 GB of RAM. The user can choose whether to boot in 32-bit or 64-bit mode, by
following these instructions on Apple's support site.
