The description is actually a bit deceiving. I believe the "sediment" filters he refers to are actually 1 (as in single tower used for this) fibre sediment pre-filter at 5 micron mesh size and then the next 2 towers are 2 medium grade carbon blocks of 5 micron particle porosity, then into the RO membrane. The last 2 are the DI and then that little carbon tube thing (referred to as a carbon block) is presumably to capture any last junk if the DI resin is expired or leaches because it wasn't changed soon enough. A useless piece so long as everything is working and maintained correctly.

One question not answered yet: You do not need a second DI cartridge once you are setup and operating correctly.