I just finished the game, romancing Calia. I had read about it, so I knew you had to answer "we're just friends". In the wiki, it's written :
"The lock-in point is very obvious".
I totally disagree with it being obvious. It certainly isn't obvious that it will lock-in the romance, even if you assume that the question is related to it. But it's not even obvious that it has anything to do with the romance. Her question isn't clearly about love. Answering "I don't know yet" could mean "I don't know yet what to think of him/if he can be trusted/etc..." especially given the very different personalities of Calia and Jespar, there's no reason to assume that she's inquiring about love.
I'm pretty certain that without having read about it, I would have answered "I don't know yet" if only because my personality leads me to make cautious and not definitive statement, and the meaning and motive of the question would have been unclear to me. I wouldn't have been sure of what she was asking exactly. And even if I had believed it was about love, I would never have guessed that this random statement could put an end to the romance with Calia, let alone locks a romance with Jespar. And I would have waited in vain for development in Calia's romance. If and when I had realized that I was locked in a romance with Jespar (I don't like much the character), I wouldn't have guessed that this had been the turning point. I probably wouldn't have remembered the exchange. I'll probably have thought that I had been too nice with Jespar and scored more "like" points with him than with Calia over the course of the game. And in a rerun of the game would have made sure to be unpleasant with Jespar to avoid the same situation. And still would have failed, because I probably would have again answered "I don't know yet", and that's what matters.
Similarly, someone wanting to romance Jespar might very well answer "we're just friends" simply because that's exactly your status at this point. Or as I wrote above thinking that "I don't know yet" means "I don't know if this man can be trusted", for instance. Hence not a positive answer wrt to Jespar but at the contrary a negative one. And it's even less obvious in this case that it's the lock-in point, because I'm not sure why you would think that an answer made to Calia could influence your relationship to Jespar. At least you have no reason to expect that it would. You would assume that if there's a lock-in point for your romance with Jespar, it would be in a scene involving Jespar.
I'm pretty sure I'm not the only one who passed the "We're just friends" test only because he had read that it was the turning point and what was the correct answer. I really think that this should be fixed, and that it should be made more obvious that this question is about relationship, and is important.