Is (1R,3aR,4S,6aS)‐1,4‐dibromo‐octahydropentalene chiral or achiral?
It indeed is achiral, because the red dot is the center of symmetry, hence it is superimposible with its mirror image.
Having an internal plane of symmetry is not the only criterion, the presence of a centre of symmetry also leads to the compound being achiral.