I think you are getting into a map/territory issue. We think of it as a spectrum because we observe a spectrum among people with diagnosable autistic disorders

I think you are getting into a map/territory issue. We think of it as a spectrum because we observe a spectrum among people with diagnosable autistic disorders. However, we do not have a fundemental understanding of what autism is, or a binary test to detect it. This would imply that in reality there is a spectrum that extends beyond the diagnosable region(s) we have.