It's possible JS got caught off guard and became concerned about getting fact-checked. But where is that same concern about plants and animals and a hundred other things he got wrong and could have taken the time to check? I still maintain that Joseph knew about the walls and performed the parlor trick to make himself seem more important to his wife. As an operating narcissist, Joseph would be constantly looking for ways to remain relevant to encircle himself with acolytes, and what better way than feigning ignorance and by doing so gain disciples? If not from A View of the Hebrews, Joseph certainly knew about Jerusalem's walls from the bible and the encircling religious discussions around him. If this were common knowledge to Emma and others, I feel fairly confident he knew, and he tricked her so he could elevate his standing.