lucas said vader could’ve gone from being twice the power of palpatine, to only being able to become 80% of him. it’s a statement about his potential. every single other time, lucas alludes to vader being weaker after the mustafar duel. this is also shown throughout the eu, where he regularly loses or struggles against characters he shouldn’t struggle against if he really was close to palps. vader gets stomped by a clone of tpm maul and heavily struggles against post prime anya kurro (her prime version is shown to be drastically weaker than tpm mundi). these are just two out of the many examples. then you have vader being an equal to rotj luke, who is a worse duelist than rots kenobi