I do think Luntz offended Trump supporters, but I also think hardcore Trump supporters are probably easily offended. Trump is hot-headed and reactionary and is legitimately guilty of some of the things Luntz accuses him of. He often says things in the heat of the moment and then has to retract his statements later. I think his closest followers and supporters will definitely be offended by Luntz's comments just like their leader was offended as we saw in his response on Twitter.
They may or may not have gotten upset when he said that, but he still was free to say those things. It is easy to understand why Trump supporters would get upset over this, the same way Hillary supporter would get upset over some of the things that are said about her.
The supporters of Donald Trump, though misunderstood by many people worldwide, are motivated by their passion for a number of issues such as gun control and immigration. By dismissing Trump's campaign, Frank Luntz also dismissed the concerns of millions of dissatisfied Americans. These supporters want to have their fears and concerns validated just like supporters of other candidates.
It doesn't matter what anybody says about Mr. Trump; both his ardent supporters and Mr. Trump himself will take offense. They probably don't mind that somebody called the campaign a joke - it is just another example of a leftist conspiracy manifesting itself, although they probably aren't able to cognate any of those words. Perhaps Mr. Trump can throw Frank Luntz in prison if he is elected.