The president was correct.
Most likely most of the non-leftists were there to support keeping Robert E. Lee's statue and were unaware that "white nationalists" were claiming to be in charge of the protest.
When I was a student, the gay activists would from time to time announce a "blue jeans day" when people were supposed to wear blue jeans to show support for the gay cause. But they did not make much of an effort to publicize it, and most people wore blue jeans most of the time anyway...so students would arrive on campus and discover that they were supposedly showing support for gays. Some people would even go home to change clothes when they learned that.
So I would guess the former Obama activist who was pretending to be a white nationalist at the time of the Charlottesville protest may have engaged in a similar kind of deceitful tactic.