spectrum is a good word because of spectroscopy, where for example a single beam of light is broken down into constituent parts

in this ASD model, a single person is like a light source, ASD traits are like frequencies, and ASD itself is like the EM spectrum

this is useful because our best understanding of ASD today is multidimensional

as you say, it is not supposed to be used as like "the spectrum" is a line from "normal" to "autistic"

unfortunately most people aren't familiar with spectroscopy, but I think it's a good metaphor

do you have a suggestion for a better word than spectrum, that could convey the same rich metaphor but be less easily misunderstood?

That metaphor actually fits well with how it is interpreted in my head. Even the "visual" of a spectroscope's graph, just turned 90º in my mind.