How can a pilot fly a distance of 594 NM connecting two VOR's?
You are correct that VOR's rarely have a range of more than 200 NM. However, Alta Floresta VOR (ATF) is colocated with an NDB (and DME). In spite of less accuracy, NDB's have a much greater range (partly because their signals can travel as ground waves). This makes it possible to have a very long segment on the G678 airway without other navigation aids. When leaving the coverage of Brasília VOR/DME (BSI), you should be able to pick up ATF NDB.