Well, tyranny is synonymous with absolute power and power unchecked. Eraqus was adamant about the light. He wanted nothing to do with the darkness, and this is reflected through Aqua when Cinderella's Fairygodmother says something along the line of "You cannot use light to fight the darkness." And Aqua being like 😮 "what do you mean⁉
In that context, only light was being used. Light, which should always be in balance with darkness, was disproportionately used. In other words, light (and the people using it) are tyrants for not keeping in balance with darkness.
I hope I'm conveying what I'm thinking correctly