No you are right about that. The THS4508 has a gain of +2, and by using the resistor pairs we do
1) Reduce the gain back to unity
2) Reduce the input DC level from 1.8V to 0.9V to match the input range
3) Terminate the signals at the input of the DRS chip to minimize reflections
I know this is not so obvious from the schematic, so thanks for asking.