I'm still not really sure whether there's actually a DMA going on - it'd just be a waste of time since all of the REU's data is already in memory (not necessarily main memory though)...
It is in fact DMA, you are programming the controller in the REU to transfer from/to REU and it can also swap or compare C64 and REU memory areas. "Memory areas" meaning whatever start address and number of bytes (up to 64k) you desire, that's one advantage of this approach.
You can also use the DMA controller to move large amounts of data inside the main memory (copy to REU, copy back from REU at different position) much faster than the actual CPU running a copy loop could. GEOS actually uses this approach to speed up the display routines, and there's at least one scene demo out there utilising this.
