Cutler certainly did not write the whole first kernel alone, although the initial size of the NT team was quite small, at least at the very beginning. The story of the development (until the at least the first release, IIRC) is in the book "Show Stopper!"; quite interesting.
Show Stopper is a wonderful book if you're interested in the historical development of large systems. It's second only to The Soul of a New Machine. I highly recommend it.
