I find this take quite challenging, although I know it is one shared by a lot of autistic people.

I understand that if a person has no support needs, they cannot be diagnosed with autism. But that person may still be neurodivergent, and therefore to me it seems to follow that you have folks who are autistic with high support needs, and folks who are autistic with low support needs. Then, you have neurodivergent folks with no support needs. But this seems to me like a difference in degree, rather than category, and which would mean that the “spectrum” analogy works quite well.

With a clear understanding that I am not trying to minimise the struggles autistic people face, a sincere desire to learn, and an open mind, would you mind trying to help me understand?

Autism is something you are born with. It is simply who you are.

Support needs can change over time. You can need less help because you learn better coping strategies and have a stable environment or you can need more as you get older. It is not fixed.

Support needs are denoted in level because that is what system like schools and the like need. They don't really map to reality. Like for example a autistic person can have really bad sensory issues, being really sensitive to sounds, restricted diet and the like but decent social skill. Another autistic person might not have any sensory issues but really struggle with social stuff. Who needs more help? They need different kinds of help.