A lot of the disagreement about autism comes from the fact that people are trying to describe a very complex condition with one or two words. Autism is a combination of communication profile, sensory profile, cognitive profile, executive function, internal distress, masking style, demand tolerance, care and support needs. Those things vary independently, so two autistic people can have the same diagnosis and completely different day to day realities.
The problem is that everyday language only gives people blunt labels like mild or severe, and those labels end up flattening the person. Mild often means the traits are less visible to others, not that the internal experience is easy.
Accurately describing an autistic profile takes more than two words. You need to include all the categories.
For example, I'm an autistic person with situational mutism, high sensory, social-communication and mental health support needs, high masking + high internal distress, high demand dysregulation, mid cognitive and executive functioning support needs, low care needs.
My daughter is an autistic person with situational mutism, high sensory, social-communication and mental health support needs, high demand dysregulation, high cognitive and executive functioning support needs, mid care needs, mid masking + high internal distress.
We're both are autistic but our profiles and support needs are slightly different. I have high, mid and low support needs. DD has high and mid support needs. Some people may have only high support needs, some only mid and others only low support needs.
So the issue is not whether autism varies in severity. It clearly does. The issue is that the language we have is too limited to capture that variation. Until there is more widely accepted multidimensional language, people will keep reaching for mild and severe because they are the only shortcuts available, even though they don't really do the job.