I think the word "spectrum" is reasonable, as it implies a broad range. Or it's analogous to the rainbow with a variety of colours.

But what people consistently misunderstand is that there is a fundamental dichotomy at the diagnostic level. Speaking from the perspective of the DSM, which I prefer because it's at least concrete and has medical relevance in North America, you meet the criteria for Autism Spectrum Disorder ("on the spectrum") or you do not ("not on the spectrum").

In other words, the diagnostic criteria themselves do not constitute a spectrum, especially not a linear one. Maybe people are confusing this with the DSM's three levels of support needs.