If one player is auto-following another for several minutes, and does nothing to contribute to the fight; they are definitely afk. The afk player was just trying to use the auto-follow feature to circumvent the afk-kick timer. You were in the right to initiate a vote kick. The afk player was angry that you caught them in the act of being afk, so they threatened to report you for the vote kick. Your use of vote kick is 100% valid, and you broke no rules.