They don't ... they have DDR3 memory attached to the ASICs / FPGAs.  If you look on the Bitmain site, you can see that the device is a) twice the size of their other miners and b) is about the hash equivalent of 4x 580's.  If you consider the 800 watts of draw, it uses more Watts/MH than a rig with 4x 580s.  About the only attractive thing about it is the price ... $800.

The only decent way to attack the Ethash mechanism is to take advantage of the 64 DAG lookup steps in the algorithm.  If you broke your DAG storage into 64 or 128 or 256 separately accessible pieces, you could pipeline the 64 lookup's and take advantage of the random nature of the lookup addresses.  The point is to do 64 lookup's at the same time taking advantage of their being in separate pieces of memory ... usually.  In this way you could pipeline the Ethash and have 64 going on at the same time with each at one point along the 64 step process ... ignoring the pre and post processing.  Occasionally, you would have a collision when two or more lookup's needed the same DAG section and so you'd stall for that step.  It's difficult to get all of that address and data bus space hooked into an ASIC or FPGA.  So it's still a hard problem ... not impossible though.