No, you did not. The mastery selection screen looks like the picture in this post. There is no way you “became” a Void Knight without hitting the giant “Choose Mastery” button.
Just reroll a new character and save the Void Knight for a future character design. You lost about 3 hours of game play, it’s not the end of the world. Asking them to completely change their game philosophy to account for your carelessness is ridiculous.