This thread title should be changed to Fe and agreeability. What Jung is really pointing out is that Fe taken to an extreme would rather forego its real opinion, simply to keep the peace. The fact that he chose to use art to illustrate the point, doesn't mean those with a strong preference for Fe don't know what beauty is. That is not what is going on here.
What if someone with a strong preference for Fe is out to dinner and rather than tell the waiter the food really stinks, they tell them their dinner is "fine." Well, surely you aren't going to suggest the person doesn't know what good food is now, are you? That would be silly.
The underlying issue is the same whether we use art or food to illustrate the point - a strong Fe preference would rather be agreeable, than disagreeable, in order to avoid conflict.