The _SFR_MEM16() is a macro to return two bytes of data of the specified address.
The _SFR_MEM16() is defined in hardware/tools/avr/avr/include/avr/sfr_defs.h as below.
#define _SFR_MEM16(mem_addr) _MMIO_WORD(mem_addr)
It calls _MMIO_WORD().