If you’re absolutely certain that you never saw a screen that looked like the one that was linked and you never attempted any sort of cheating. Not saying you did, I can just see how someone might accidentally set it while trying as a possibility of what happened. Kinda the only explanation I can think of really.
Then you should submit this as a bug report. It shouldn’t be possible to become any of the mastery classes without selecting it from a screen like the one linked in the thread.
It does warn that it is a permanent choice at least once, maybe two or three times if you read all the dialogue. I still do see people missing that so it probably needs to be really really in your face more.