I've lived in California my entire life, and attended public schools my entire life, including UCSB undergrad and UCB Boalt Hall law school. At every level of my education, we discussed the causes of the civil war, and covered other countries that abolished slavery without war. There was nothing remotely controversial about questioning why the political climate in the US was different than that in Great Britain or Brazil.
As a side note, your description of "Berkeley liberals" is insulting. Particularly since the majority of the mayhem has been caused by armed out-of-town right wing extremists.
In any event, the "controversial" part of Trump's interview was that he made the statement, "People don't realize, you know, the Civil War, you think about it, why? People don't ask that question. But why was there the Civil War? Why could that one not have been worked out?" in the context of Andrew Jackson, who was dead at the time, being "really angry that he saw what was happening with regard to the Civil War, [and] he said, "There's no reason for this.'"
A couple things. First, Andrew Jackson was dead. He didn't see what was happening with regard to the Civil War, and he never said, "there's no reason for this" from his grave. That's alarming because we haven't had a President of the United States before that lacked the historical education and civics knowledge of a pre-teen.
Second, it's not a novel question. It's not an exaggeration to say that poems, books, lectures, classes, plays, mini series, and movies have all been dedicated to the question of why there was a Civil War, and why "that one could not have been worked out." Again, it's shocking that the President of the United States has never contemplated this question before, and is raising it as thought it were for the first time.
Third, his manner of speaking about a topic as grave as the Civil War, particularly at a time when people are blindly loyal to a political party more out of tribalism and cultural identity than an educated, intellectual decision, is alarmingly cavalier and ignorant.
I don't know if the above is "controversial," as much as it is shocking.