Trump has said many times in past and present interviews that he has wanted to run as a republican in the elections because he believes that republicans are stupid people. Throughout his campaign he has treated his supporters like complete idiots and continues to use that belief to insult his followers while swaying votes.
Trump knows what he is doing. He knows what gets prejudiced people to like him and he's very savvy about it. He knows that he can do anything and they will still support him because they are mindless and blind sheep. He does not believe his supporters are smart and he plays off of that.
For years, Trump has questioned whether Barack Obama was really born in the U.S. and was the leader behind the "birther campaign." His recent statements that he had changed his theory - eventually recanted so he could "get on with the campaign" - were probably an effort to restore any negative publicity, and, in turn, win votes. Furthermore, Trump supported his most recent words with the claim that he instead would rather talk about jobs, the military and working to defeat ISIS.
One statement taken out of context can not justify claiming Trump thinks his supporters are stupid. There is nothing wrong with wanting to move and leave behind that which is not beneficial. If the conversation was not leading in a positive direction, Trump has every right to change the direction. It is his campaign after all.