Holy shit, this is an awful tweet. High and low functioning autism are medical terms used to describe how someone can FUNCTION. How well they can carry out tasks such as use the bathroom, feed themselves, and, yes, work.
[A thread:]

While the distinction was made originally by Hans Asperger, a nazi doctor, the research continued to be built on and reviewed after his death and it is agreed on by the medical community that yes: some autistic people can function at a higher level than others.

As for how this distiction fits into society today, it's incredibly important for the sake of people with mental disorders and differences that there are distinctions made between these conditions.