It's the hardware peripheral available on MEGA2560 micros, called "external memory interface". The WR, RD, ALE pins are manages automatically, no need to set/clear these pins by program. It is designed for connecting more SRAM directly to the uC.
In the code, display memory is then addresses with pointers, exactly the same way as internal RAM. I have used that a long time ago to interface displays with 8051 micros; it should work here also.
Anyway, by using the latest Adafruit library, it is faster than before (~2x).