> If it's a spectrum, everyone is on it somewhere.

No, because the endpoints of the spectrum are not defined as 0% autistic and 100% autistic.

The spectrum definitionally only includes people diagnosed with autism.

Your approach is like saying "there is a 'how bad is the cancer' spectrum" where 0 is "no cancer" as opposed to something like "cancer but easily curable." No reasonable definition of "cancer suffering spectrum" would include "doesn't even have cancer."