Yes...and No. I had heard this story before about King going extemporaneous in the dream speech....and found this link after reading your comment.
I'm no expert on King, but it seems as if he had a 'typical' speech writing team (for this event anyway). The great part about the 'dream' speech is that he apparently abandoned the pre-written speech spontaneously. As a result, we were given one of the greatest speeches of all time. That part, although not written down initially, was all King.
http://www.forbes.com/sites/carminegallo/2013/08/27/public-speaking-how-mlk-improvised-second-half-of-dream-speech/