I think you are being a bit pedantic with the wording. I think all value is subjective. Even money. But I can still say "overvalued". I would argue that he is not the one to determine the value of a post but rather he is one to determine the value of a post.
Whales have been flagging for disagreement of rewards for over a year now. I disagreed with flagging for that reason because it is based on aggregate earnings of a user over many other up-voters when the flagger has nothing against the post itself. I find I agree with you.