Yes, this limits the utility of ptrdiff_t. But I don't believe you're allowed to subtract arbitrary pointers in C anyway. So you might need a >63-bit char array in order to legally hit this limit, and you probably don't have 263 of RAM.

Practically, amd64 is already limited to more like 48 bits (4 levels of 9 bit pagetables describing 12 bit pages) of memory.