I could be wrong, but I think what the poster is getting at is that when it's shared as mild/severe, those with less familiarity view it as a linear progression. The more accurate representation I've seen from autistic self-advocates is that if a sound mixer where there's multiple faders (needs) that all exist at once at different levels depending on the individual.
Stereotypically speaking, a non-speaking autistic is often considered to have "more severe" autism (in quotes because that phrasing is often not well taken in the community), but it's possible that their other faders Are set in a way that theme park lines are fine for them. Where a speaking autistic may stereotypically be considered more "mild" in presentation, yet their other faders (needs) are such that being in line is not possible.
So it's not about saying there aren't different levels of need within a diagnosis - of course there are, but that wording like mild/severe is often stereotyped in ways that a person's needs may be more likely to be misconstrued or misunderstood/inappropriately classified.