Not the exact wording but the general premise behind it is a fair counter point in any disagreement. When someone is attempting to gain a higher moral authority, bringing up any hypocrisy is a reasonable thing to do. If pointing out hypocrisy is then dismissed, it is reasonable to assume the other person is not arguing in good faith and therefore should no be taken seriously.
Pretty much. Whataboutism could be any other bad thing, Tu Quoque is “you do too”, so essentially you do the same bad thing.