Although perhaps insignificant, since symmetries give at maximum one order difference, Tromp and Farneback did not account for symmetries. Also it seems difficult to implement this using their approach.
Is it possible to reduce symmetries using only the final nr of positions found by their algorithm? Simply dividing by eight is not correct, since most positions have less symmetries.
Correct. Note that since current superko rules do not restrict symmetrical positions, but only identical positions, accounting for symmetry in legal positions is counterproductive for calculating the bounds on the possible number of games.